Shark Conference 2000
Honolulu, Hawaii February 21-24
|ASSESSMENT AND MANAGEMENT REQUIREMENTS TO ENSURE SUSTAINABILITY OF HARVESTED SHARK POPULATIONS|
Shark populations generally have low productivity and can be depleted rapidly, thus requiring special management and research if they are to be harvested sustainably. Consultative, management and research mechanisms involving fishers, fishery managers and scientists need to be quickly established and have the capacity to implement effective fishery management and data collection systems within appropriate time frames. Shark fishery assessment requires time series of catches and abundance indices and reliable estimates of demographic parameters that represent growth, mortality, and natality. Shark species undergoing long migrations and exhibiting complex stock structuring require application of models that allow for spatial- and stock-structure and for the use of tag data for estimation purposes. This might require incorporating the results from separate fine-scale models of shark movement dynamics designed to represent alternative hypotheses of movement. In addition, there is a need to determine parameters that take account of the selectivity characteristics of the fishing gear and to address biases in demographic parameters and abundance indices caused by length-selective fishing mortality and sampling bias. Critical habitats such as pupping, nursery and mating areas and migration lanes also need to be identified and might need special protection.
Reported landings of chondrichthyan fishes currently exceeds 700,000 tonnes per annum. Most of the landings are evenly divided between sharks and batoid elasmobranchs (rays and skates), with a small component consisting of chimaeras (Anonymous 1996). Chondrichthyans provide about 1 per cent, and sharks about 0.5 per cent, of the world's fisheries products.
Fisheries for sharks are common throughout the world but the overall number of species harvested is small compared with the number of species of teleosts and invertebrates harvested. The sharks are taken with a variety of types of fishing gear but most are taken by gill-net, long-line and trawl in industrial and artisanal fisheries. Small amounts are taken in traditional and recreational fisheries, and in shark control programs designed to reduce the risk of shark attack at bathing beaches.
Every part of sharks has been used for some purpose. However, in some fisheries only the meat is retained, while the rest of the animal is discarded; in other fisheries, only the fins, or liver or skin or cartilage are retained. Given there is a growing demand for shark products while many stocks are in decline, there is a need to change these wasteful patterns of usage (Walker 1998). At present, only a few countries have developed shark fishery management plans or implemented shark fishery controls (Bonfil 1994; Shotton 1999a).
Shark and other chondrichthyan populations tend to have low productivity and therefore need careful management. Only a small proportion of the biomass can be taken each year sustainably from the population of a species with low productivity. Such species tend to be long-lived and to have low natural mortality and low natality rates. However, as within any group of animal, the productivity of sharks varies widely between species but the productivity of sharks as a group tends to be less than the productivity of invertebrate groups and teleosts. The productivity of sharks is perhaps higher than that of marine mammals.
Nevertheless, those characteristics that make sharks vulnerable to overexploitation also provide for stable populations and stable fisheries. Young sharks are born large-sized which makes their survival and recruitment rates less likely to fluctuate widely from year to year in response to environmental variation than the survival rates of the relatively small-sized eggs, larvae and young of invertebrates and teleosts.
Stock assessment of any shark fishery requires monitoring catch extraction and catch composition, provision of time series of indices of abundance, and parameters accounting for the selectivity characteristics of the fishing gear and species' demography. It also requires an understanding of stock structure and patterns of movement, and, for some species, estimates rates of movement between the main regions of the fishery.
This paper outlines the basic management and stock assessment requirements for shark fisheries. It also highlights some of the special management requirements of shark fisheries and of the assessment problems associated with shark fisheries. The outline is consistent with the International Plan of Action for Conservation and Management of Sharks, which is designed to be consistent with the United Nations Food and Agriculture Organization Code of Conduct for Responsible Fisheries, international law and the Conference on Straddling Fish Stocks for Highly Migratory Fish Stocks.
Countries engaged in fisheries management should adopt measures for the long-term conservation and sustainable use of shark fishery resources. Conservation and management of each fishery taking sharks, whether at local, national, subregional or regional levels, should cover the entire area of distribution of the fishery and be based on the best scientific evidence available. These measures should be designed to ensure the long-term sustainability of shark fishery resources, and short-term considerations should not compromise these objectives.
The International Plan of Action requires each regional management body to develop an Individual Plan of Action and each nation to develop a National Plan of Action by early 2001. These plans should specify the actions needed for the conservation and management of sharks. Their implementation requires a minimum set of institutional arrangements and recurrent activities.
National and regional institutional frameworks
Institutional, policy and legal frameworks are required at the national level to govern access to shark resources. The costs, benefits and effects of alternative management options designed to rationalize shark catches and levels of fishing effort need to be evaluated. Effective consultation between fishery managers, representatives of the various fisheries sectors, fishing communities, other stake holders, and scientists is required to ensure appropriate input into fishery management planning, development and decision-making processes. Fisheries policies should aim to minimize conflict among extractive shark fisheries resource users and between these extractive users and the non-extractive users of shark resources. All of this requires raising public awareness of the need for the management of shark resources.
Effective shark fishery control, surveillance and law enforcement measures, inspection schemes and vessel monitoring systems need to be implemented. Such measures should be promoted and, where appropriate, implemented by subregional or regional fisheries management organizations.
Facility for research, monitoring and, where necessary, observer programs in support of shark fishery assessment and management and of species conservation are required. In addition to research on the resources and monitoring the fishery, the effects of species interaction, climatic, environmental and socioeconomic factors should be investigated. The data need to be managed in effective databases to ensure efficient verification, validation, retrieval, reporting and archiving. The data and results of such research should be available to all interested parties.
The responsibilities of existing bilateral, multilateral and international fishery management bodies need to be extended to include, or give higher priority to, shark fisheries. Where necessary, to manage international shark fisheries, new regional agreements might need to be established. Special conservation and management measures should be implemented for trans-boundary shark stocks, straddling shark stocks, highly migratory shark stocks, and high seas shark stocks throughout their ranges.
National and subregional or regional fisheries management organizations and arrangements should agree on the means by which the activities of such organizations and arrangements will be financed. Cognisance needs to be taken of the relative benefits derived from the shark fishery and the differing capacities of countries to provide financial and other contributions. Where appropriate, and where possible, such organizations and arrangements should aim to recover the costs of management, monitoring and research.
Countries participating in regional fisheries management organizations should implement internationally agreed measures adopted in the framework of such organizations, which are consistent with international laws. These measures should be designed to deter the activities of vessels flying the flag of non-members or non-participants, which engage in activities undermining the effectiveness of conservation and management. Existing regional bodies that are initiating efforts to collect information about sharks include the Inter-American Tropical Tuna Commission (IATTC), the International Council for the Exploration of the Sea (ICES), the International Commission for the Conservation of Atlantic Tunas (ICCAT), the Latin American Organization for Fishery Development (OLDEPESCA), the Indian Ocean Tuna Commission (IOTC) and South Pacific Commission (SPC).
Vessel registers and fishing licences
Fishing vessel registers provide a depository of information on fishing vessels, companies, gear, licences and individual fishers. These registers can be used to obtain required data on the fleet by complete enumeration as a legal requirement. Required data on vessels include vessel type, vessel size, gear type, country of origin, fish holding capacity, engine horse power, navigational aids, and number of fishers.
Fishing, fish processing and marketing companies should also be registered. Fishing company registers should include data on number of vessels and details of vessels and fishing gear. Fish processing and marketing companies should provide data on type of processing, type of fish and capacity of processing and marketing.
Operators of fishing vessels and fishing gears should be required to hold a valid fishing licences. Unlike vessel registers, fishing licences tend to be issued for access to specific fisheries for a set period of time. Because licences have to be periodically renewed, they provide a way of updating information on vessels and fishing gear.
Vessel registers and licensing systems are complex and require well-established administrative procedures supported by effective data communications, data storage and processing components. Being such they have certain types and size of fishing units such as industrial and semi-industrial fleets. These systems provide a basis for complete enumeration of catch and fishing effort. Small-scale and subsistence fisheries involving large numbers of fishing units, on the other hand, are often not part of a registering system. Monitoring these fisheries requires enumerators at key sampling sites.
Complexities of managing sharks
The stock status is unknown for most harvested populations of sharks. The main reasons for this lack of knowledge are the relatively low world production, historically low economic value and the small number of shark species compared with the large number of highly valued harvested teleost and invertebrate species. Other reasons include the difficulty and high costs associated with at-sea work required for studying shark populations.
Sharks taken in fisheries targeted at sharks where catches or effort are controlled can be harvested sustainably, but the number of species targeted is small which creates difficulty in assessing shark stocks. Most sharks are captured in multi-species fisheries directed at more productive and more highly valued teleost species. Harvest strategies designed to maximize economic benefits from these multi-species fisheries will inevitably deplete the less productive shark and other chondrichthyan species without management actions designed to prevent this. In some fisheries, by-catch and non-target catch reduction methods will have to be developed and implemented.
At present, because most of the shark catch is taken by fishers targeting high valued teleost species, most of landed non-target catch and discarded by-catch is reported as unidentified shark or mixed fish or not reported at all. This lack of species identification for catches and lack of information on fishing effort means basic data for fishery stock assessment are available for only a few species (Shotton 1999b). There is an urgent for the development of field guides to enable species identification from whole animals, carcasses and, possibly, fins, skins, vertebrae and heads.
A large number of sharks and other chondrichthyan species require 'special protection or management'. As the quantities of most species of shark have not been reported accurately, species could be at high risk of depletion without it being recognized. 'Critical by-catches' are by-catches of species or populations that are in danger of extinction, and 'unsustainable by-catches' are by-catches of species or populations that are not currently at risk but would decline at current levels of by-catch (Hall 1996).
'Special protection or management' is a term adopted for a species requiring special protection or management because of its poor conservation status or rarity. This term avoids terms such as 'endangered', 'threatened', 'vulnerable' or 'depleted species' or 'in danger of extinction'.
Some countries have adopted definitions for some of these terms, which have legal standing in their jurisdictions, and some international organizations have published classification criteria for the conservation status of species, but, as yet, there is no single set of criteria accepted by all nations (Hall 1996).
Several species have already been identified as requiring special protection or management. Carcharodon carcharias, widely distributed in temperate waters, is recognized as having its populations markedly reduced in several regions of the world over the past three or four decades. As a top-level predator its abundance is naturally low compared with many other species of shark and its productivity appears to be low. C. carcharias occurs most commonly close to shore where it is easily targeted and caught; the large animals in particular are easy to catch when they are feeding near seal-breeding colonies. Younger animals often tangle in fishing gear. The decline in abundance of this species, indicated by records from game fishing (Pepperell 1992) and bather protection programs (Dudley 1995), has led to its being protected in South Africa, Australia and several states of USA. Similarly, populations of the spotted ragged-tooth shark (Carcharias taurus) off New South Wales, Australia, have had to be protected because its populations were reduced by spearfishing and long-line fishing (Pollard 1996). Several countries are either implementing or considering special protection or management for basking shark (Cetorhinus maximus) and whale shark (Rhincodon typus).
Some of the most threatened species of shark occur in freshwater habitats. One reason why these species are more vulnerable than those inhabiting marine waters is that the amount of freshwater in rivers and lakes is small compared with the amount of seawater on Earth. These areas are more accessible to exploitation and more prone to habitat degradation than marine waters, particularly given the tropical rivers and lakes where freshwater species occur mostly in developing countries with large and expanding human populations (Compagno and Cook 1995). At least three species of 'river shark' (Glyphis spp) are now extremely rare (Compagno 1984). Batoids such as Pristis spp. found in fresh water are also threatened.
Dogfishes (Squalidae) inhabiting the continental shelves and the continental slopes are taken as non-target catch or as discarded by-catch in the large demersal trawl fisheries of western Europe, USA, Canada, South Africa, New Zealand and south-eastern Australia. Some species (notably Squalus acanthias and S. megalops) found on top of the continental shelves have withstood trawl fishing but, like many of the teleost species studied from the deeper and colder waters of the continental slopes, the deepwater dogfishes are particularly vulnerable to the effects of fishing because of their low productivity. Dogfishes produce fewer young per pregnancy and are longer-lived than many other groups of shark. The slopes are usually steep, and the total area of associated seabed is small compared with the areas on top of the continental shelves and on the abyssal plains of the oceans. As some species of dogfish are confined to narrow depth-ranges on these slopes, the total areas occupied by these species are small. Expansion of demersal trawl fisheries into progressively deeper water to target high valued teleosts on the continental slopes in some regions of the world is placing several species at high risk (Walker 1998). In New South Wales, Australia, for example, fishery independent surveys undertaken 20 years apart indicate that several species of dogfish (Centrophorus uyato, C. harrissoni, C. moluccensis, Squalus mitsukurii, Deania quadrispinosa) and one species of chimaera (Hydrolagus ogilbyi) inhabiting the continental slope have already been severely depleted. Other species (notably Squatina spp and Raja spp) occurring also on top of the continental shelf were less severely depleted. Only Squalus megalops showed no sign of depletion; its abundance possibly increased (Andrew et al. 1997).
The effects of habitat change on fish populations are generally more difficult to study and historically have been less intensively studied than the effects of fishing. Nevertheless, changes in abundance of neonatal and juvenile Galeorhinus galeus are evident in inshore and coastal waters of southern Australia (Stevens and West 1997). Critical habitats such as pupping, nursery and mating grounds and migration lanes identified for several species of shark (Branstetter and Musick 1993; Castro 1993; Ripley 1946) and parts of the ranges of some species with restricted distributions might need special protection.
Catch landings, catch discards, catch composition, fishing effort and abundance data should be collected continuously or at intervals sufficiently frequent to provide time series data. There are several data collection methods that can be applied. It is essential these data are well managed and readily available.
Catch landings and discards
Total catch in numbers or weight needs to be recorded or estimated because it represents removal of biomass and individuals from the ecosystem; the catch is the fundamental impact fishing has on fish populations. Where both weight and numbers are recorded, mean weight of shark in the catch can be determined.
Catches should be broken down by species, location and date. Further breakdown by sex and length of shark (or broad size category or maturity) enable application of sex-based and length-based stock assessment techniques. Discarding of sharks in dead or in poor condition has important biological implications and should always be recorded or estimated. Total catch consists of total landings and discards.
Sharks are usually headed, gutted and finned before landed ashore so appropriate weighting factors are required for conversion to live weight equivalent units (also called whole or round or green weight) or to a standard partial weight equivalent. This means that there needs to be provision for reporting the form of the sharks (e.g. whole, headed and gutted carcass with fins on, headed and gutted carcass with fins off, fins only or liver only). Discard estimates can sometimes also be obtained from fishers; otherwise on-board observers are required during fishing trips to record information on discards and fishing locations.
Fishing effort can be related to fishing mortality and it is necessary to relate it closely to specific gear use for stock assessments. Units of fishing effort for gill-nets are kilometre-lifts or kilometre-hours which requires recording total length of gill-nets and soak time. Similarly units for hooks are hook-lifts and hook-hours which requires recording total number of hooks set and soak-time. Soak time becomes invalid after baits are lost from the hooks. Soak times for gill-nets and long-lines with baited hooks have three phases: setting period, hauling period, and period between. Times for distinguishing the three phases should be recorded. Units for trawls are kilometres trawled, which can be determined from vessel positions at the start and end of each haul or from trawl time and average trawl speed. Species or group of species targeted should be recorded because this affects catch per unit effort (CPUE).
Height and mesh-size of gill-nets, hook-size, average distance between baited hooks, and trawl net dimensions and codend mesh-size should also be recorded. Power of vessel and presence of navigational aids such as Global Position Fixing and colour echo sounders are relevant to fishing power of vessels. Recording species or group of species targeted allows fishing effort to be treated as targeted effort or non-targeted effort in stock assessments.
Size and age data for male and female sharks, separately, provide information on stock structure. Male and female sharks and other chondrichthyans can be readily distinguished by the presence of claspers on males. In fisheries, where fishers remove claspers at sea but leave the pelvic fins in tact on females, the animals can be sexed, but, where the pelvic fins are removed from both males and females, the animals can only be sexed by on-board observers.
Size composition data can be collected by sampling vessel catches. This requires a standardized length measurement for recording lengths of shark. Because sharks are usually headed, gutted and finned, the length measurement has to be a partial length. If the position of junction between caudal fin and body trunk is readily identifiable then the longest reliable partial length on the trunk that can be measured is from the posterior edge of the last gill-slit to the base of the tail. An alternative to the last gill-slit is the anterior margin of the base of the pectoral fin or, where pectoral fins are removed, the anterior margin the pectoral girdle. Other positions on a trunk used for defining partial lengths are the bases of dorsal fins. Where sharks are landed in size categories, it is necessary to sample all categories and to apply raising procedures that lead to accurate estimates of length composition in the catch.
Conversions from partial length of landed carcass to total length or fork length of shark are required to present data in terms of total length or fork length. If it is necessary to adopt more than one standard length measurement, the data should be converted to a single standardized length, ideally total length or fork length.
Age composition of the catch is determined from length-at-age data. This involves determining ages of sharks of known length within each of a number of length-classes covering the full size range of the animals in the catch to provide an age-length key. Age-length keys can be combined with the length-frequency composition of the catch to determine the age-composition of the catch. Ageing chondrichthyans involves counting growth-increment bands in sectioned or whole vertebrae or in other hard parts such as sectioned spines. Ideally age-length keys are determined each year because of continual changes in the age composition of the population.
Indices of abundance
Fishery stock assessment requires a time series of an index of abundance that can be related to stock size. For shark stocks these can be provided from fishery CPUEs and from fishery-independent survey data.
CPUE or catch rate is a valuable index for long-term monitoring of the fishery and is often used as an index of stock abundance, where some relationship is assumed between the index and the stock size. However, CPUE alone can be an unreliable index of stock size. There can be a problem with changes of fishing efficiencies or operational patterns over time; hence, routine surveys of fishing gear and navigational aids should be adopted. Also, CPUE can be misleading in a fishery where fishers targeting aggregations of fish can provide high CPUE while the stock declines rapidly (hyperstability), or, conversely, fishers removing highly vulnerable aggregations in an otherwise diffusely distributed population can cause CPUE to decline much more rapidly than stock abundance (hyperdepletion). CPUE should be separate for each stock unit, fleet and gear type. Where there is complex stock structuring the data need to be spatially disaggregated. Some of the problems of differences in efficiency between vessels and changes in areas fished from year to year can be adjusted by standardisation of CPUEs using generalized linear models; however, standardizations require careful statistical consideration of the residual (error) structures of the data.
Fishery independent survey of fish density with a standard vessel using standard fishing gear can avoid some of the biases inherent in fishery CPUE data. Such surveys can be costly and require careful design, particularly if a stratified sampling design is adopted to improve precision.
Surveys carried out using institutional research vessels or commercial fishing vessels at the level of species, stock or sub-stock can provide indices of stock abundance and distribution. These surveys require fishing with a standard vessel using standard fishing gear at predetermined fishing stations selected according to a fixed-grid, fixed-site or stratified random sampling design. Such surveys should provide, firstly, an estimate of average fish density (per area swept by a trawl net, or as fish encounters with long-lines or gill-nets) over the entire spatial range where the stock(s) might be found, and, secondly, mapping of the spatial distribution of density over the entire range. Areas of habitat occupied by the fish should also be estimated.
Many countries undertake regular fishery-independent survey of their trawl fisheries and regularly analyse the data for the more valuable teleost species but most of data available for sharks and other chondrichthyans have not been systematically analysed. Several exceptions include South Africa (Compagno et al. 1991), Argentina (Cousseau 1986), Uruguay (Ehrhardt et al. 1977a; Ehrhardt et al. 1977b) and New Zealand (Hanchet 1986; Walker et al. 1999). Similarly, fishery-independent surveys of tuna and tuna-like fishes have produced valuable by-catch data on sharks, but most of these data have not been analysed.
The feasibility of adopting fishery-independent surveys using bottom-set gill-nets for providing a time series of indices of relative stock abundance is being explored in the shark fishery of southern Australia. A pilot survey undertaken during late 1998 provided the basic data required to design a full-scale survey whereby selected vessels would be engaged to undertake fishing with a standard set of gill-nets at selected fixed-sites at a selected times of the year. The pilot survey provided data on inter-shot variability as a basis for determining the number of sets of the gear needed to give sufficient statistical power to detect changes in annual catch rates over time at various levels of probability. The pilot survey also helped determine appropriate spatial distribution of the sets and the cost of such a survey (Anonymous 1998; Prince et al. 1999; Punt 1998).
A similar fixed-site sampling scheme was undertaken in Tasmania during 1991-97 and in Victoria during 1994-97 for providing abundance indices of newborn and young juvenile sharks in school shark nursery areas. Variance in the catch rates was found to be too high to detect a change in abundance from year to year during the sampling periods. Whilst the data provide valuable baselines for comparisons made in the future, the approach is considered impractical for monitoring pre-recruits (Stevens and West 1997).
Scientific research is required to determine various parameters and variables for shark populations and their habitats, and for the fishing gear used to harvest sharks. Such research can be carried out using institutional research vessels or commercial fishing vessels. In addition to fishery-independent survey, scientific research methods can address various objectives, which need to be addressed at the level of species or, in some cases, stock or sub-stock.
In a gill-net fishery, an objective is to determine how selectivity of gill-nets varies with length of shark and mesh-size of gill-net, or, in a hook fishery, an objective is to determine how selectivity of hooks varies with length of shark and hook-size. Determining selectivity parameters requires experiments using gill-nets with a range of mesh-sizes or using hooks with a range of hook-sizes. In a trawl fishery, an objective is to determine whether by-catch abatement devices can be developed to reduce the catch or kill of shark species.
Laboratory procedures need to be developed for ageing sharks from, depending on species, whole or sectioned vertebrae or from other calcified parts such as sectioned dorsal spines present in some chondrichthyan groups. Also depending on species, the visible clarity of growth-increment bands in these structures require chemical staining or special illumination or microradiography to enable interpretation for age estimation.
Various life-history parameters need to be determined from seasonally collected biological data on sex and length-frequency composition, maturity stage of ovaries and oviducal glands, number and size of ova, and number and size of in utero eggs and embryos. These data are required for determining the relationship between the number of young produced at each pregnancy and maternal length. They are also required for determining the proportion of the female population breeding each year at length. The relationships of number of young versus maternal age and proportion of females breeding versus age can be determined if age data are collected for the animals sampled for reproductive data.
Some species of shark have sites where they aggregate for mating or giving birth, or they predictably travel along certain migration routes to these areas. It is important to identify these sites and routes as it might be necessary to provide special protection from fishing through closed areas or closed seasons to protect the breeding fish in the population. Furthermore, species with well defined nursery areas, where the newborn and young animals are found, may need special protection from the effects of fishing and habitat degradation. The nursery areas are often in shallow inshore areas where they are vulnerable to the effects of habitat change caused by industrial, domestic and agricultural development in coastal and catchment areas. Also, aquaculture, ecotourism, spread of exotic organisms and pollution in the marine environment, and, possibly, in some regions of the world, global warming and ozone thinning might impact on the nursery areas. These changes should be carefully monitored.
Tagging programs can be adopted for estimating growth, mortality and movement rates. Sharks can be successfully tagged with internal tags inserted through the body wall into the coelomic cavity (body cavity), with rototags inserted through the bases anteriorly of dorsal fins, and with dart tags inserted between the basal cartilage of the dorsal fins. Dart tags inserted into the muscle tissue in many species have low retention rates. Whenever tagging programs are undertaken, some double tagging should be undertaken to estimate tag retention rates. Also, injecting sharks with oxytetracycline or other hard tissue stains provide a basis for validating assumptions on the periodicity of growth-increment bands in shark vertebrae and other hard parts adopted for ageing purposes. Estimates of tag reporting rates should also be made. Archival tags are beginning to provide insights into the diurnal vertical migration behaviour of sharks.
Genetic studies undertaken in conjunction with tagging studies are required to better understand stock structuring. Mitochondrial and nuclear microsatellite DNA studies might be useful for detecting maternal philopatry, multiple paternity and cryptic species (Heist 2000). In addition, genetic studies are enabling development of procedures for distinguishing several species at a time from tissue samples to address identification problems arising form heading, gutting and finning sharks (Shivji 2000).
Unaccounted fishing mortality
'Unaccounted fishing mortality' (Anonymous 1995) can be significant in some shark fisheries deploying gill-nets; this is less significant in hook and trawl fisheries. Fast swimming species, such a Galeorhinus galeus, dependent on ram-jet ventilation of their gills for respiration, tend to die more quickly than bottom-dwelling species when captured by gill-net or hook. Bottom-dwelling species, such as Mustelus antarcticus, Cephaloscyllium laticeps and Heterodontus portusjacksoni, with well developed spiracles to aid gill ventilation are better able to ventilate their gills after capture by gill-nets and can struggle vigorously, thereby either escape the gear or become more tightly enmeshed. Dead sharks not tightly enmeshed can drop out of gill-nets and contribute to 'unaccounted fishing mortality' through 'drop-out fishing mortality'. Sharks eaten by other fish or mammals after capture in the gear contribute to 'unaccounted fishing mortality' through 'predation fishing mortality', and dead sharks either partly or totally decomposed or eaten by invertebrates when fishing gear is left in the water for extended periods also contributes to 'unaccounted fishing mortality'. 'Predation fishing mortality' resulting in a retained or discarded damaged catch and a cryptic catch lost before the fishing gear is recovered. Lost gill-nets contribute to 'unaccounted fishing mortality' through 'ghost fishing mortality' until the gill-nets are rolled into a ball by tidal flow.
The stock structures of shark populations need to be determined. Stock structuring by size, age, sex and reproductive condition occurs for many species of shark, and the recapture of tagged sharks at positions long distances from the positions of release indicate that many species are highly migratory. There is also evidence of mixing between genetically distinct populations.
Stock structuring has implications for approaches to stock assessment and for how a fishery might be managed. A single population with high rates of mixing between separate regions of a fishery (whether the population be homogenously distributed or highly structured) needs to be assessed and managed as a single stock. However, a single homogenously distributed population with low rates of mixing between separate regions of a fishery, might be more appropriately assessed and managed as separate regional sub-stocks. Spatially-separate populations should normally be assessed and managed as separate sub-stocks, but the fleet dynamics of the fishery might necessitate the fishery be managed as a single unit. In shark fisheries where there is mixing between separate breeding populations, multi-stock models are required. Furthermore, where the rates of mixing between different regions of a fishery are high, spatially-structured models that incorporate movement rates between separate regions of the fishery are required for stock assessment.
Stock structuring is not well understood for most species of shark. Complex structuring in widely distributed species has been hypothesised for Prionace glauca (Casey 1985; Gubanov and Grigor'yev 1995; Nakano 1994; Stevens 1990), Squalus acanthias (Compagno 1984; Ford 1921; Hanchet 1986; Hisaw and Albert 1947; Ketchen 1986; Nammack et al. 1985; Saunders et al. 1985) and Galeorhinus galeus (Walker 1999). G. galeus populations in several parts of the world are genetically different, and while southern Australia and New Zealand stocks are genetically different (Ward and Gardner 1997), tag release-recapture data indicate mixing between these two populations (Hurst et al. 1999; Walker et al. 1997).
Complex stock structuring together with the practice of fishers targeting more than one species complicate interpretation of CPUE and, without spatial disaggregation of the data, can cause CPUE to be an unreliable index of relative abundance. This problem can be demonstrated in the shark fishery of southern Australia, where the fishers usually target one of two species-the highly aggregated G. galeus, which forms large migrating schools and feeds on schooling fish and cephalopods, or the dispersed Mustelus antarcticus, which feeds on dispersed epibenthic fauna. These species are very different from each other in that G. galeus exhibits much more obvious spatial stock structuring than M. antarcticus. In this fishery, complex stock structuring of G. galeus requires the stocks to be assessed using a spatially-structured multi-stock models with spatially-disaggregated data (Punt et al. in press). On the other hand, there are advantages assessing the stocks of M. antarcticus as several separate sub-stocks (Walker 1994b). This is because of differences in targeting practices in different regions of the fishery and the effects this has on CPUE and because the species has fairly low mixing rates between the various regions of the fishery.
Stock assessment of G. galeus in this fishery demonstrates that the application of spatially-aggregated models can give highly uncertain results. Allowing for spatial- and stock-structure and using tagging data for estimation purposes, the mature biomass at the start of 1997 is estimated at 12-18% of the 1927 level (Punt et al. in press). This is a much narrower range than that obtained using a spatially-aggregated model and ignoring the tagging data, which estimates the 1994 mature biomass at 15-46% of the 1927 level (Punt and Walker 1998).
The hypothesis adopted for the assessment applying the spatially-structured model requires there to be separate breeding sub-populations but there be mixing at other life history stages. One way of explaining mixing sub-stocks for G. galeus is to invoke the concept of philopatry ('home loving') effected through 'natal homing' whereby pregnant female sharks return to their birth place (Hueter 1998). If this hypothesis is correct 'stock' can be defined as 'a group of animals that have the same pupping grounds and same movement patterns'. If the females from different stocks mate randomly with males then 'stock' is likely to be memory based rather than genetically based (Punt et al. in press; Walker et al. in press).
The results of most tag release-recapture studies are presented by plotting information for only the recaptured animals and ignoring the uncaptured animals. The approach considers the start and finish positions and time at liberty of each animal plotted on a map to provide a visual representation of the magnitude of displacements and rate at which displacement occurs (i.e. velocity) by individual animals (Hurst et al. 1999; Olsen 1953).
Vector analysis can provides a basis for comparing populations between separate areas for a species or between species on the basis of quantities such as mean displacement, distance, velocity and speed. However, such analyses provide information only on the shortest distance between release and recapture positions rather than on the full distance moved between these positions (Walker 1983; Walker et al. 1997). A major disadvantage of vector analysis (and simply plotting the data) is that no account is taken of the spatial distribution of fishing effort.
Another approach involves estimating rates of movement between separate regions, where the rate of movement is the proportion of animals leaving one region to move to another region within a specified time-step. This approach treats the contribution of each tag independently and makes use of information from both the recaptured and non-captured tagged sharks. Data inputs to the model include total fishing effort within discrete time intervals for each type of fishing gear, the gear selectivity function of each fishing gear deployed in the fishery, and region, shark length and date at the time of release and the time of recapture (Dow 1989; Walker et al. in press; Xiao 1996). Tag release-recapture programs are usually too limited in scale to determine the number of movement parameters required for a fully spatially-structured stock assessment model.
These three approaches should be viewed, therefore, as a basis for developing a range of feasible alternative movement hypotheses, which can be developed as simulation models tuned on the basis of assumed parameter values. These parameter values can be either fixed or used as starting values for estimation in complex spatially-structured stock assessment models. This approach was adopted recently for a stock assessment of G. galeus off southern Australia (Punt et al. in press; Walker et al. in press).
Stock, recruitment and natural mortality
Implicit for any stock-recruitment relationship is that a fish population is limited in size, whether fished or not, and is held at some more or less fluctuating level by natural controls. Density-dependent natural mortality could affect abundance of either the existing adult stock or the young it produces (Ricker 1954). In the past, fisheries scientists have assumed recruitment independent of stock size and were generally more concerned with preventing growth overfishing than recruitment overfishing. Recruitment is now generally viewed as largely independent of stock size during the developmental phases of a fishery, but the fishery will reach a point where recruitment begins to drop because of overfishing (Hilborn and Walters 1992).
Simulations of harvested populations of M. antarcticus with models that incorporate the demography of sharks, parameters to represent the selectivity of fishing gear and the history of fishery catch and effort with density-dependent responses in natural mortality provide insights into stock-recruitment relationships. These studies suggest that the Beverton and Holt stock-recruitment relationship applies to this species. Whether density-dependence applies to all age-classes or to only several of the youngest age-classes remains uncertain (Walker 1998).
The assumption of constant natural mortality with both age and time does not appear to be valid for sharks. This assumption is necessary for most teleost and invertebrate fisheries where there is usually little information on the pre-recruited age-classes. However, sharks are different because information on reproduction can be used for calculating the number of offspring a population can produce. Application of available estimates of natural mortality for M. antarcticus (Walker 1992) and G. galeus (Grant et al. 1979) for the recruited component of the population in fully age-structured models for M. antarcticus (Walker 1994a; Walker 1998) and G. galeus (Punt et al. in press; Punt and Walker 1998) indicates that natural mortality of the pre-recruited component of the population has to be several times higher than the natural mortality estimates available for the recruited phase of the population. If the natural mortality were not high, there would be far too many births for the population to reach equilibrium. Adopting an asymmetric 'È-shaped' mathematical function allows natural mortality to vary with age such that it decreases rapidly with age during the first few years of life and then increases with age during the later years of life. The initial decrease in natural mortality with age allows for higher survival associated with improved capacity to escape predators and to catch prey, as a result of increased size. The subsequent increase in natural mortality allows for the natural process of senescence (Walker 1998).
Fishing mortality and gear selectivity complexities
A species' catchability is affected by its biological characteristics and the characteristics of the fishing gear. Pelagic and semi-pelagic species that actively swim in the water column (notably Carcharhiniformes, Lamniformes and Hexanchiformes) are more likely to encounter a gill-net or baited hook and therefore likely to have a higher catchability than sluggish species of shark (notably Squatiniformes, Pristiophoriformes, Heterodontiformes, Orectolobiformes and Squaliformes) and batoids that can rest on the seabed. These bottom-dwelling species, on the other hand, are more vulnerable than the more powerful swimming species to demersal trawling.
Fishing gear selectivity gives rise to a range of complexities that relate to the dynamics of harvested shark species. Selectivity by trawl nets for size of shark has not been described, but the effects of gill-net mesh-size selectivity (Kirkwood and Walker 1986) has shown to be stronger than the effects of hook-size selectivity (Walker 1983) for size of shark in Mustelus antarcticus. For both gill-nets and hooks, sharks of different sizes are not equally vulnerable to capture. Depending on mesh-size, small sharks swim through gill-nets whereas larger ones are more vulnerable to capture. Beyond a length of maximum vulnerability they are progressively less vulnerable as they cannot so readily penetrate the meshes of the nets (Dudley 1995; Kirkwood and Walker 1986; McLoughlin and Stevens 1994; Nakano and Nagasawa 1996; Simpfendorfer and Unsworth 1998). These effects of size selectivity are stronger for fusiform-shaped sharks than for more dorsoventrally-flattened species or for species with protruding structures such the heads of hammerhead sharks (Sphyrnidae), the rostral teeth of saw sharks (Pristiophoridae) and sawfishes (Pristidae), and the dorsal spines of dogfishes, horn sharks (Heterodontidae) and chimaeras (personal observations).
Length-selective fishing mortality resulting from the effects of gear selectivity combined with high fishing mortality in a fishery can cause marked distortions to growth curves determined from length-at-age data. Such distortions, referred to as the 'phenomenon of apparent change of growth rate', are caused by length-selective fishing mortality (Lee 1912; Ricker 1969). In a gill-net fishery, for example, the probability of a slow-growing shark being caught in a gill-net of particular mesh-size during its life-time is higher than the probability of a fast-growing shark. This is because a slow-growing shark is vulnerable to capture for a longer period than a fast-growing shark. Furthermore, at any instant, the length-frequency distribution of each age-class in the surviving population depends on past levels of fishing effort. Length-selective fishing mortality from gill-nets of mid-sized mesh-sizes have the effect of culling a greater proportion of the fast-growing sharks than the slow-growing sharks from the young age-classes and of culling a greater proportion of the slow-growing sharks than the fast-growing sharks from the old age-classes. This can have the effect of overestimating the magnitude von Bertalanffy parameters and and to underestimate the von Bertalanffy parameter . The distorting effects on growth curves by hook fishing effort in a fishery are not so marked (Walker et al. 1998).
Length-selective sampling bias can result when length-selective fishing gear is used for collecting biological data from a wild shark population. Growth curves, for example, can be distorted when the length-frequency composition of sharks in a sample collected for age determination in any age-class is not representative of the length-frequency composition of sharks in that age-class in the wild population (Dow 1992; Ricker 1969).
Age and growth uncertainty
Age and growth uncertainty is another problem. Apart from sampling bias and length-selective fishing mortality, choice of method for shark age determination can lead to various biases in growth curves. Counting growth-increment bands in sectioned vertebrae without careful discrimination of major bands from minor bands can overestimate age and the von Bertalanffy parameters and and to underestimate the von Bertalanffy parameter . Conversely, counting bands in whole vertebrae can underestimate age, and counting growth-increment bands from different positions along a vertebral column can give different age estimates (Officer et al. 1996). In Galeorhinus galeus, for example, there is a marked difference between the growth curves determined by counting hypermineralized growth-increment bands on microradiographs of sectioned vertebrae of sharks collected in southern Brazil (Ferreira and Vooren 1991) and counting growth-increment bands on the external surface of alizarin-stained whole vertebral centra from sharks collected in southern Australia (Moulton et al. 1992). Although some of the difference observed between these curves might be explained by regional differences between southern Australia and eastern South America, a major part of the difference is probably attributable to the method of ageing.
Correcting growth curves for the effects length-selective sampling biases, length-selective fishing mortality and perhaps age measurement error requires development of stochastic models of growth. These models are required because they better represent heterogeneity in length-at-age of fish or heterogeneity in growth length-increment of tagged fish than do simple deterministic models (Ricker 1969; Troynikov 1998; Troynikov and Walker 1999).
Artificial selection for fast or slow growth in a harvested shark population could be effected through the choice of mesh-size. The effect of the higher probability of catching slow-growing sharks over fast-growing sharks, in the long-term, might provide artificial selection for faster growth in the population. Hence, there may be economic benefits in a shark fishery by adopting mesh-sizes designed to target sub-adults and to avoid the larger breeding fish. Conversely, adoption of larger mesh-sizes catching breeding sharks might provide for artificial selection for slower growth.
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