Alexia C. Morgan and
George H. Burgess
Florida Museum of Natural History
Division of Fishes
University of Florida
Fishery dependent data collection is one of the most resourceful tools available to fishery managers. However, the management plans put into effect based on this type of sampling will be only as good as the data collected. It is critical that managers determine what are the most important data to be collected and implement some system of data recording before signs of overfishing occur. One of the biggest mistakes fishery managers make is waiting until the populations are in peril before initiating a management plan. This Section provides a wealth of information on what type of data should be collected in a shark fishery, why and what methods can be used for data collection.
11.2 CATCH ESTIMATES
11.2.1 Why and how to collect catch data
Fisheries resource managers must rely on several important factors in determining the status of a fishery. Among these factors are catch estimates for both target species, any bycatch involved in the fishery or of all species in a multi-species fishery. Each individual fishery should maintain a continuous database that includes all reported catch, estimates of discards, and estimates of non-reported catch. Catch estimates can be obtained in a variety of ways including fishery observers, logbooks and dockside and shoreside monitoring. Each of these monitoring systems is discussed in more detail later in this Section.
Catch estimates are used to illustrate the species composition of individual fisheries, utilization rates, monitor quotas, estimate fishing mortality and to calculate catch per unit effort (CPUE). These estimates include not only what is sold at port, but also that which is discarded or used as bait, retained for personal consumption or transferal by the vessel's crew. In other words, all fishes retained or discarded should be documented. This type of information becomes extremely important in fisheries where quotas are used as a management tool. Catch estimates allow managers to determine the current status of a fishery and whether the quotas have been met, are being underutilized or if catches are exceeding the limits. The data produced from catch estimates can also be used to show historical trends in the fishery, commonly used to build quota systems and to estimate population abundance. These numbers can also be integrated into models to predict the outcome of future management plans or what effect current management will have on the stock.
Catch estimates are critical and can be a contentious shark fishery management issue in countries with well-developed fisheries and fishery management regimes. Catch data often come to fishery managers from captains vessel owners or shoreside merchants who, understanding that high catch figures might lead to management resulting in reduced future catches, are prone to under report the actual catches. However, in areas with government-run fisheries, the opposite may be true as fishers and marketers are inclined to demonstrate higher productivity to their superiors. In individual transferable quota fisheries, fishers may over report their catch in order to ensure a large individual quota. Since managers who determine the status of a fishery use these data, under or over-reporting can result in inappropriate or unfair management measures, such as unreasonably high quotas and can lead to overfishing, which ultimately negatively affects all stakeholders. It is imperative that every effort be made to monitor the accuracy of all catch estimates.
11.2.2 Catch disposition
In areas where not all the catch is marketed, at-sea monitoring provides the most accurate catch data. At sea, fishery observers should accurately record the number of individuals by species, note whether the shark is alive or dead when landed and record the final disposition of each shark brought aboard a vessel. Disposition is the final fate of the shark, (e.g. saved for market, used for bait, discarded live, discarded dead, discarded after removing fins, etc.). Codes should be made for each possible disposition on field data sheets, that are both easy to use and to remember; commonly, initials or letters are used that correspond to each type of disposition.
Disposition estimates for individual species allow fishery managers to better understand what is actually happening in the fishery. For example, in the U.S. Atlantic shark fishery, several hammerhead species are commonly caught but not landed because their flesh is not marketable. Therefore, the catches of these species do not appear in market or dockside data sets. Disposition data taken by marine observers allows fishery managers to acknowledge the cryptic mortality incurred by all species caught and can help detect declines in abundance. At-sea catch estimates often give a different view of what is actually happening in a fishery than landings (i.e. marketed catch) data. However, in areas where the entire catch is brought back to port, landings data accurately depict the scope of total fishing mortality, but not the gear-induced fishing mortality.
Bycatch is a common side effect of directed fisheries. Its level depends upon the type of gear employed and amount of effort expended. Sharks are commonly caught as bycatch in a number of directed fisheries such as the oceanic tuna and swordfish longline fisheries, inshore and offshore gillnet fisheries targeting mackerels (Scombridae), herrings (Clupeidae) and other species, and shrimp trawl fisheries. The catch numbers, mortality and disposition for all of these sharks must be recorded in the same manner as in directed and multi-species fisheries.
11.3 CATCH PER UNIT EFFORT (CPUE)
11.3.1 Definition of CPUE
Catch per unit effort (CPUE) is a ratio commonly used to eliminate temporal and regional trends in simple estimates of fish stock abundance. The “catch”portion of the measure may be expressed as the number or weight of the entire catch, a selected subset of the catch or a particular species in the catch. The “unit effort”portion of the rate usually refers to the time a uniformly designed and employed piece of fishing gear is deployed in the water. In the absence of uniform gear use, CPUE can be applied on a coarser scale utilizing whatever effort data are available. Units of effort depend on the type of fishing gear used and can use (in increasing levels of fine-scale reliability) such measures as the numbers of vessels, vessel-days, gillnet or longline sets or number of hook, trawl or gillnet hours. Many aspects of the fishery can be monitored utilizing CPUE analysis, including: trends in overall fishery catch rates, catch rates of target versus bycatch species, catch rates in specific depth strata, seasons or subregions, catch rates of size classes and sexes, and catch rates of specific vessels or types of vessels.
CPUE is a much more powerful tool than catch data alone. A decline in CPUE over a time period is usually a good indication that stocks are declining. However, advancements in fishing gear, improvements in fishing abilities of captains and crews and changes in fishing grounds, current patterns or weather can influence CPUE trends. Interpretation of CPUE data, therefore, must be undertaken with knowledge of such potentially contributing factors. See Section 10.6.6 for further discussion of CPUE.
11.3.2 How to collect CPUE data
126.96.36.199 Gillnet fishing gear
The important characteristics of gillnet gear include total net length, mesh size, number of panels, panel length and depth, water depth at deployment, deployed depth in the water column (bottom, midwater or surface set), orientation of the set (parallel or perpendicular to shore or current), and soak time (time the gear is in the water) (Figure 11.1). The type of information fisheries managers are seeking from CPUE data dictates the catch and unit effort measures used to calculate CPUE. The following are examples of possible CPUE calculations:
Three variations in the placement and design of gillnet fishing gear. The net floats and anchors are all visible (NOAA).
Catch rate of female sharks caught per panel hour. For this calculation, we must know the total hours the gear was in the water during the entire fishing period and how many panels were used (Figure 11.1). Three variations in the placement and design of gillnet fishing gear are possible. The net and anchor floats are all visible on the water during that time period and how many female sharks were caught during the time period is known. Consider a situation in which the total fishing hours was 300, the total panels fished was 5 and total number of female sharks caught was 10. Unit effort is calculated by multiplying the total hours (300) by the total number of panels (5), resulting in 1500 panel-hours of effort. The female catch (10 sharks) is then divided by the panel-hours (1500) resulting in a CPUE of 0.0067 females per panel-hour. If a CPUE measure is a small number, as in this case, the CPUE's numerator and denominator are often multiplied by an exponent of 10 (e.g. 1000) to produce a larger and more easily expressed CPUE numerator. For example, if our CPUE of 0.0067 female sharks caught per panel-hour is multiplied by 1000, the result is a more readily understood catch rate of 6.7 sharks caught per 1000 panel-hours.
Catch rate of sharks vs. other species in 100 mm mesh panels. Here one must know the total hours the 100 mm panel gear was in the water during the entire fishing period, the catch of sharks in these panels during that period and the catch of other species in the panels during the same time period. If 2000 kg of sharks and 4000 kg of bycatch species were captured during 1000 hours of fishing, the calculated CPUE of sharks would be 2.0 kg an hour (2000/1000) of fishing of 100 mm panels and the CPUE of other species would be 4.0 kg an hour of fishing of 100 mm panels (4000/1000).
188.8.131.52 Longline fishing gear
Longline gear characteristics include mainline length, gangion length, number, size and types of hooks, water depth at deployment, where deployed in the water column (bottom, midwater depth or surface set), orientation of the set (parallel or perpendicular to shore or current) and soak time (Figure 11.2). As with gill nets, the types of catch and unit effort measures used by fisheries managers to calculate CPUE are based on the specific information they are seeking. The following are examples of possible CPUE calculations:
Catch rate of sharks taken in depths of 25–50 m. To calculate the catch rate of sharks per hook-hour, one must know the total number of sharks captured while fishing in depths of 25–50 m, the total number of hooks used while fishing in this depth range and the total time the gear was in the water in this depth range. Assume 12 sharks were caught on 100 hooks fishing for 12 hours. The fishing effort is then 1200 hook-hours (100 hooks × 12 hours) and the CPUE is 0.01 sharks a hook-hour (12/1200), which also can be expressed (after multiplying by 100/100) as 1.0 shark per 100 hook-hours fishing at depths of 25–50 m. CPUE expressed as catch per hook-hour or by number of hooks are the preferred measures of expressing longline CPUE; the alternative, catch per set, is less useful because the number of hooks and set time varies from vessel to vessel and from set to set.
Diagram of pelagic longline gear (National Marine Fisheries Service).
Catch rate of sharks taken by an artisanal fishing village during the month of January. Sometimes minimal data are all that are available for calculating CPUE. For instance, consider a situation where the only measure of catch is an artisanal fishing village's monthly sales of fins. Having obtained that number, a crude CPUE can be calculated even if only minimal effort data are available. An estimate of the number of fishing vessels can be derived from vessel counts at the port. Thus, CPUE based on 700 kg of fins originating from 7 vessels fishing in January would yield a CPUE of 100 kg of fins per vessel per month. Effort data might be refined if interviews of fishers revealed that those vessels fished only five days a week throughout the month. That information would produce a new effort estimate of 140 vessel-days (7 vessels × 5 days × 4 weeks) and a more meaningful CPUE of 0.5 kg of fins per vessel-day (700/140). If a count of the fins sold also is available, then an estimate of the number of sharks caught can be made after interviewing fishers to learn the number of fins that are harvested from an individual shark. If four fins are routinely taken from a shark and the 700 kg of fins represented 1120 fins, then the January catch was 280 sharks (1120/4) and a more refined CPUE of 2.0 sharks per vessel-day (280/140) can be obtained.
184.108.40.206 Trawl fishing gear
Trawl CPUE is usually determined as the catch per hour of bottom trawling time. Variables that affect trawl CPUE include mesh size, length and width of net, distance between trawl doors, lengths of bridles and foot rope, length and depth of float line, time of trawling, presence of a turtle excluder device (TED), bycatch reduction device (BRD), beam, “tickler chain”or rollers, and cod end mesh size and configuration (Figure 11.3). Standardization of gear type employed, trawling speed and time of trawling greatly increases the accuracy of CPUE estimates. Non-standardized trawl gear and methodologies can result in considerable variation in CPUEs, and biased data (see Section 12).
A shrimp boat rigged with otter trawl gear. Floats, nets and doors are all visible (NOAA).
220.127.116.11 Purse seine fishing gear
Catches by purse seine nets depend on their circumferential length, depth, mesh size and ability of the fishing crew (Figure 11.4). CPUE is usually calculated as the number of sharks caught per set, but, as for trawl gear, standardization in gear type greatly improves comparisons of CPUEs.
Fishing vessel pulling in a purse seine net. The floats, net and circular enclosure are all visible (NOAA).
11.4.1 Landings reports
Landings reports are one part of the process of estimating total catch and are also used to show how many of each shark species are brought to port for distribution or sale. There is often quite a difference in the number of sharks caught and the number of sharks actually landed. Historical landings data can be used to correlate increases and decreases in certain species landings to changes in the local market and export demand. Management plans that use quota systems often use only the reported landings against the quota. This is a biased assessment of the actual catch; because many sharks may be discarded at sea, there is commonly under-reporting (and occasionally over-reporting) of the landed sharks and sharks are difficult species to identify. A well-designed management plan will use both catch and landings data.
11.4.2 Problems associated with species identification
A major shortcoming in using landings data is the common lack of species identification. In many shark fisheries, the sharks are dressed at sea to insure high quality of the flesh. Properly dressing a shark involves removing the head, fins and entrails as soon as possible after being caught (often after bleeding the shark by removing the caudal fin at the caudal peduncle (see Section 14) (Figure 11.5). This makes it nearly impossible to accurately identify sharks to species at landing. If proper identification is not made at sea, then the landings reports will only reveal the total number of sharks caught and cannot be used to show trends in species abundance. There are a few exceptions to this, including the landings of sharks with tell-tale external coloration or morphological features such as tiger, leopard, whale, blue, white and mako sharks. Some regional guides for the identification of carcasses (often called “logs”) or fins may be available.
Dressed carcasses ready for sale in the U.S. (Florida Museum of Natural History).
Landing carcasses also eliminates the ability to record the total size or weight of a shark. Measurements of sharks at the dock after they have been dressed do not provide accurate estimates of “wet”weight, i.e. the whole animal weight. Fishermen use different dressing techniques and so measurements of a carcass will not provide a true indication of the size of the shark, but rather the style or ability of the fishermen. However, this can be solved by developing length relationships by species and relating interdorsal distance to total length (see Moutopoulos and Stergiou (2002) for examples of length-length relationships in teleosts). The weight of landed sharks is more easily measured on shore than at sea, but trying to convert from dressed to whole weight can be tricky because conversion factors may vary between fishers and over time. In addition, sex and reproductive maturity cannot be determined after the shark has been dressed. Quantification of bycatch is also lost using landings data, as is information on cryptic mortality (e.g. freshly-caught sharks used as bait at sea) and vitality (alive or dead) of captured sharks.
Landings data are easy to obtain because they are obtained on land, and there is usually more space and equipment available and the sharks are dead. However, because of the limitations noted above, landings records offer a restricted amount of pertinent information and should be used with discretion.
11.5 FISHING MORTALITY
Fishing mortality is an important but sometimes under-reported aspect of fishery dependent monitoring. The more than 400 species of modern sharks have evolved from multiple phyletic lines, occupy a wide range of habitats and engage in a variety of life styles concordant with their morphological and physiological attributes. Individual species react differently to being hooked or ensnared in a net. The respiratory mode of a species —in particular, whether a species uses ram-jet ventilation (and thus must constantly be in motion to respire) or can actively pump water over their gills— is the largest single factor affecting survival time after initial capture in fishing gear, but pre-existing physiological stress, ontogenetic stage (size) and soak time are factors as well.
The condition, alive or dead, of every shark that is caught, whether targeted or taken as bycatch, should be recorded. This condition does not refer to the final fate of the shark, rather to its status —alive or dead— when initially removed from the fishing gear. There are a number of shark species, notably the tiger (Galeocerdo cuvier), blue (Prionace glauca), sandtiger (Carcharias taurus) and many orectolobiform species, including the nurse shark (Ginglymostoma cirratum) that typically survive longer than other sharks when taken on a longline hook or in a gillnet. In some regions these species are considered of low market value and often are returned alive to the sea. By contrast, species like the dusky (Carcharhinus obscurus) and hammerhead (Sphyrna spp.) sharks have short survival times when captured in fishing gear. Managers that rely solely on landing-data and, or catch estimates without considering the at-vessel fishing mortality of all species may be inclined to overlook the need for management of a significant segment of the fishery. Knowledge of the high fishing mortality these species endure may affect a fishery regulator's choice of management measures. For example, at the time of writing, the dusky shark was prohibited from being landed in the northwest Atlantic waters of the United States. This regulatory measure, which might appear to be a well-considered tool enacted to eliminate fishing mortality, actually is largely ineffective because about 70% of longline-caught dusky sharks are dead by the time the fishing gear is retrieved. This type of management measure, therefore, has a limited effect on conserving the dusky shark population and an alternative strategy aimed at keeping fishers away from dusky concentrations should be considered. Section 13 provides further discussion of management measures.
11.6 FISHING AREAS
Development of preferred fishing areas depends upon vessel size and cruising range, the availability of targeted species and size classes, weather, currents and bottom configuration. Recording accurate fishing locations associated with catch data allows fishery managers to distinguish geographical variability in catch rates, denote changes in the activities of the fishing fleet and determine sub-population differences in life history parameters of target and bycatch species. Significant declines in regional catch rates should be examined carefully because such trends often are indicative of localized over-fishing.
Recording fishing locations
The most specific and preferred way to report fishing location is by recording the latitude and longitude of every set. Usually those coordinates are recorded as gear first enters the water, at the point all gear is deployed and effective fishing has begun, as retrieval of gear begins ending effective fishing and at the time all gear is returned to the vessel. Total water depth, fishing depth and time of day also should be recorded for each of these four events. The last is of critical importance in calculating accurate fishing effort. Recording the locations and depths at times of release and retrieval is important even when anchored gear, such as some longlines and gillnets, are employed because these gears often are moved by currents and waves, or they may be picked up intentionally or mistakenly by other vessel operators and dropped off at a different location. Similarly, a single bottom trawl may cover a range of water depths and varying seafloor topography. This information is entered into a database and can be plotted to show all the locations and depths where sets are made.
Different methods for recording locations
Most commercial fishing vessels from developed nations have GPS or LORAN systems on board. For those that do not, a hand-held GPS can be used to determine location. Biologists working aboard vessels in regions where such gear is routinely absent must determine the best way to record an equivalent form of this information. For nearshore fisheries, distance from shore and landmarks such as shore structures, islands, rock formations, inlets or channels can be used to provide an approximate location. In the Maldives, fishers locate their local fishing grounds by counting the number of oar strokes from port (Anderson, 1993). When monitoring fisheries from shore, interviews with fishers may reveal which fishing grounds, reefs or banks were visited on a given trip. If only fishing range of a vessel or fleet is known, a semi-circle originating from the home port can be constructed using that range as the radius.
Catch time series
Catch time-series from fishing areas are monitored to determine stock changes. Prime fishing grounds, such as the banks and reefs, are exceptionally vulnerable to overfishing because they are easy to find, may host a variety of harvestable species and may support multiple target fisheries. Fishers exploit such areas until catch rates drop so low that they are forced to move to new locations. Nursery areas, critical regions where young sharks congregate, also are prime fishing sites because of shark abundance and relative ease of capture. These fishing areas are extremely susceptible to fishing pressure and regionally-specific management is required to prevent localized over-fishing. Management measures used to alleviate these problems may include area closures, seasonal closures and regional fishing area quotas (Shotton, 1999).
A detailed analysis of fishing locations used by a specific fleet using catch or CPUE time series often shows clear trends. These changes may simply reflect natural temporal variation in shark populations or may be directly attributable to the effects of fishing mortality. They also may be the result of changes in fishing practices, such as moving fishing effort to new target species or different size classes, altering fishing gear and increases in the fishing ability of fishers. If major changes are observed, further analysis must be undertaken to determine which factors are important. Catch estimates, changes in fishing practices, landing reports, market values and export data are useful clues used in determining the influences of change.
11.7 SPECIES IDENTIFICATION
11.7.1 Importance of accurate species identification
Accurate identification of individual shark species is one of the most important and difficult aspects of fishery-dependent sampling and is integral to good fishery management. Many directed fisheries target a large suite of species and bycatch of sharks in other fisheries often involves multiple species. Many groups, especially the requiem sharks of the genus Carcharhinus (Carcharhinidae), some triakids (particularly Mustelus spp.), catsharks of the family Scyliorhinidae and squaloid sharks (spiny dogfishes and their kin) often look similar to the untrained eye and even experts may have difficulty in identifying some species. The skates and rays (Batoidea) are also difficult to identify and there are many species still awaiting formal scientific description (see Section 3). In many areas these difficulties in species identification have lead to aggregated data simply recorded as “shark”(for all chondrichthyans), as “shark”or “ray,”or as only slightly narrower categories such as “large shark”and “small shark”, e.g. “tiburon”and “cazón”in Mexican shark fisheries (Bonfil, 1997). Vernacular names of sharks frequently vary between geographic regions and should not be the only form of identification used in catch recordings in the Maldives, the vernacular name of sharks varies from island to island (Anderson, 1993). Use of the Latin binomial (“scientific name”) —genus and species— eliminates any confusion between regional vernacular names. Every effort should be made to make sure that the total catch of all sharks —be they targeted or bycatch— are correctly identified to species level.
Shark catches need to be reported at the species level to facilitate better fishery management. Lack of species-specific data has forced many countries to report national catches and, or manage their sharks using designated multi-species groups. Japan reports its shark catch in three broad groups, “pelagic,”“benthic”and “coastal”. These groups reflect the fisheries targeting them, namely tuna longline, trawl and “other”fisheries, respectively (Nakano, 1999), rather than biological similarity. The United States places 39 species into three groups, “large coastal,”“small coastal”and “pelagic”. Individual shark species, primarily taken in the pelagic longline, bottom longline and drift gillnet fisheries are placed in these management groups based on their broad habitat preference and similarity of appearance. As fishing effort increased, it became evident that certain species could not withstand the same fishing pressure as others within a management group, resulting in sharp declines of certain species revealing the inherent difficulty associated with managing a multi-species fishery. Recording of species-level data and associated advances in understanding of biological attributes of the affected species now allows fishery biologists to fine-tune the management process.
11.7.2 Problems with species identification
Lack of species-specific data collection forces fishery managers to use aggregated shark data in their analyses. This can lead to mismanagement because of the large variation in life history patterns exhibited by individual shark species. The lack of species-specific reporting is a global epidemic in shark fishery management. According to the United Nations Food and Agriculture Organization (FAO) records, in 1966 15.8% of reported world shark and ray landings were identified to species level, 30.9% to genus and 53.7% to order (Shotton, 1999). Thirty years later, only 8.8% of the world catch was reported to species level, 18.4% to genus and 55.3% to order. Six entire FAO reporting regions, the Atlantic West Central, Eastern Indian Ocean, North Eastern Pacific, Eastern Central Pacific, Western Central Pacific and Southwestern Pacific, did not report any catches to species level in 1996 and the six countries that lead the world in reported chondrichthyan landings (Indonesia, India, U.S.A., Pakistan, Mexico and Taiwan) did not report any catch at the species level. Of those, only the U.S.A. reported at the genus level.
11.7.3 Materials used for species identification
Prior to the start of a shark fishery or as soon as possible after its start some type of species identification reference guide should be made available to fishers, observers, fish marketers and any others who will be responsible for recording catch or landing data. Identification guides vary in complexity based on the diversity of species present or captured in a region, the difficulty in distinguishing similar species, the level of education or training of the intended audience and the resources available to the author producing the guide. Section 3 provides a list of some regional identification guides.
These guides are readily usable by trained fishery observers or other working biologists, but consideration should be given to developing a simpler layout for use by fishers and marketers (Table 11.1). Most useful for field use are abbreviated, identification-only publications printed in a small format or as laminated cards such as Casey (1964), Schwartz and Burgess (1975), Castro-Aguirre and Perez (1996) and Castro (2000 a, b). Books or large guides are too bulky and too complicated for most fishers and marketers, who are not prone to devote much time to leafing through large volumes in order to identify a species. Lack of literacy is a problem in many regions as well. An alternative means of increasing the quality of identification is providing appropriate-sized posters outlining the key differences among species. Such posters can be posted on the wall of a cabin or wheelhouse aboard a vessel or in fish markets. If taking a guide or poster to sea is not practical because of limited vessel size, fisher illiteracy, or fiscal restraint, data takers should receive introductory identification training of the shark fauna they will encounter.
Example of the simple layout for species identification (Florida Museum of Natural History).
|SHARKS OF THE GENUS CARCHARHINUS|
|species||inter, dorsal ridge||1st dorsal fin||dorsal fin placement||snout.eyes||notes|
|longimanus||large, round. whitish tip||origin well behind rear tip of pectoral fin||white tipped dorsal fin distinguishing characteristic|
|falciformes||origin over tree rear tip of pectoral fin||inner margin of 2nd dorsal longer than height of fin/last 3 gill slits over pectoral fin|
|obscurus||curved rear edge of D1||origin over midpoint of the inner margin of pectoral fin||similiar to perezi, check teeth for distinguishing characteristic|
|galapagensis||straight rear edge of D1||origin over free rear tip of pectoral fin|
|perezi||origin over free rear tip of pectoral fin|
|signatus||origin of D1 over pectoral axil|
|altimus||origin of D1 over pectoral axil||snout equal to, or longer than width of mouth|
|plumbeus||snout shorter than width of mouth|
|leucas||large||origin above middle of pectoral fin||snout short, rounded /small eyes|
|limbatus||large||origin over midpoint of the inner margin of pectoral fin|
|brevipinna||origin over free rear tip of pectoral fin|
|acronotus||origin over free rear tip of pectoral fin||snout w/ dusky smudge at tip|
11.7.4 How to collect species-specific data
To facilitate data taking, a unique species code should be assigned to each shark taken in the fishery. Simple combinations of the first letters of the genus and species or the universally accepted vernacular name are easy to remember and to record quickly (Table 11.2). Requiring data recorders to write an entire shark name on a data sheet is too time consuming and will result in missing or faulty data. As noted above, the use of vernacular names is discouraged unless the name is uniformly used throughout the recording area.
11.8.1 Importance of size structure in shark fisheries
The sizes of all sharks in the catch should be consistently and accurately recorded. This can be an arduous task and may be unrealistic for some fisheries, but, such data are critical. Many species of sharks have shown dramatic population declines when certain size or age classes were targeted in Australia. During the 1940s intense fishing for adult school sharks lead to severe reductions in abundance and a subsequent change in the fishery. Fishers were forced to move further offshore and further from home to catch sub-adult sharks to make up for the loss of the adult population (Walker, 1999). Temporal shifts in the size of the catch can signal over-fishing, but this may also be the result of changing fishing practices. Before becoming a prohibited species the dusky shark in the western North Atlantic was a target of both recreational and commercial fishers. Specimens from all size classes were heavily targeted, which consequently lead to one of the most dramatic population declines in recent history.
Species codes used in a commercial shark fishery observer programme.
|Common name||Scientific name||Florida Museum of Natural History code||FAO code|
|Large coastal sharks|
|Caribbean reef||Carcharhinus perezi||CS||CCV|
|Scalloped hammerhead||Sphyrna lewini||SL||SPL|
|Great hammerhead||Sphyrna mokarran||SM||SPK|
|Smooth hammerhead||Sphyrna zygaena||SZ||SPZ|
|Small coastal sharks|
|Shortfin mako||Isurus oxyrinchus||IO||SMA|
|Longfin mako||Isurus paucus||IP||LMA|
|Bigeye thresher||Alopias superciliosus||AS||BTH|
|Common thresher||Alopias vulpinus||AV||ALV|
|Big-eyed six gill||Hexanchus nakamurai||HV||HXN|
|Seven gill||Heptranchias perlo||HP||HXT|
|Six gill||Hexanchus griseus||HG||SBL|
|Smooth dog||Mustelus canis||MC||CTI|
|Spiny dog||Squalus acanthias||SA||DGS|
|Roughskin spiny dogfish||Squalus asper||SR|
|Florida smoothhound||Mustelus norrisi||MN||MTR|
11.8.2 Fisheries targeting size classes
Many fishers target specific size classes of sharks, while others are forced to do so because of management regulations such as time and area closures or size limits. Size limits are an effective way to protect selected age classes from overfishing, but the size of the sharks being taken in the fishery must be known to determine the potential benefits of the measure. The regional market demand for sharks often is size specific. In Mexico, for example, sharks are sold as either “cazon”(>150 cm) or “tiburon”(<150 cm) and receive different prices per kilogram (Bonfil, 1997). Prevailing weather patterns also can force fishers to set their gear repetitively on certain fishing grounds, which may lead to an increase in the catch of certain size classes.
11.8.3 Recording weight and morphological measurements on land and at sea
Recorded weights of landed sharks are also used to show trends and shifts in the fishery. Most fisheries measure the quantity of landed sharks as dressed weight in tonnes (i.e. units of 1 000 kg). Landing weights often are used as surrogate indicators of catch increases and decreases. This can be misleading if the sizes and numbers of sharks being caught are not reported as well. In the absence of numerical data, potential shifts in the size composition of the catch will be missed.
A variety of measurements are taken on sharks, including fork length, total length, pre-caudal length, first dorsal rear insertion to pre-caudal pit, eye to eye (for hammerhead species) and other miscellaneous measurements (Section 3). The three most frequently used measurements are fork, total and precaudal length. When only a single measurement can be taken, fork length is the choice of most shark biologists because it provides a consistent measure of body length. All those who record data should employ consistent modes and units of measurement; the metric system is preferred internationally.
It is not unusual to find the tip of the upper lobe of the tail damaged or missing owing to a previous injury, or as the result of shark-on-shark scavenging prior to retrieval of fishing gear. Or the upper lobe may be cut off by fishers immediately upon being brought onboard. In these cases alternative measurements should be taken. A good alternative when the caudal fin is damaged or missing is to measure from the tip of the snout to the precaudal pit, the distinct notch located just anterior to the caudal fin. A measurement from the rear base of the first dorsal to the precaudal pit also is useful, especially if only butchered carcasses (i.e. heads, tail and fins removed) are available. If tail amputation removes the caudal pit, a measurement from the rear margin of the first dorsal fin to the anterior insertion of the second dorsal fin is a good substitute. For each species taken in the fishery, one should take several of the measurements as noted on each of at least 30 individuals to develop statistically significant correlations between those measurements. These relationships allow fishery biologists to convert an alternative measurement into a missing desired primary measurement, for example fork, total or precaudal length.
If sharks are landed at market whole, measurements can be made at that time. However, in many fisheries sharks are processed at sea and measurements must be made prior to finning and gutting. In some circumstances, sharks come aboard a vessel or are unloaded too quickly to measure each shark and thus only an estimated length can be made. Estimating lengths should be done only as a last resort, but is sometimes the only option. For example, observers monitoring the U.S. Atlantic directed shark drift gillnet fishery estimate shark lengths (to within 30 cm) while they are still suspended in the net (Carlson and Lee, 2000). A meter stick or other measuring device can be placed on the gunnel where the sharks come aboard the boat as a means of reference.
Obtaining the weight of a whole shark at sea is difficult, time consuming and often unobtainable due to logistic considerations. A hanging scale can be used for smaller shark species, but this is usually not a viable option for larger sharks. For this reason, most biologists weigh the whole shark or carcass at the dock or simply estimate the weight. A major problem in dockside weighing is that any sharks used for bait or discarded at sea are not weighed. In addition, since only butchered carcasses are landed in many fisheries, whole body weight data is unobtainable. Generating statistically significant length-weight curves for major species early in the monitoring process is important because these relationships allow one to convert subsequent length data into biomass estimates. An alternative is to develop relationships between different lengths and between length and weight from fishery independent surveys where all the catch can be accurately measured and weighed. Kohler, Casey and Turner (1995) provides for length-weight regressions for several species of common sharks.
Sexual segregation of sharks based on depth, season, area and sexual maturity is common in some species. The Atlantic sharpnose shark (Rhizoprionodon terraenovae) and spiny dogfish (Squalus acanthias), common species in the northwest Atlantic, aggregate by sex. Pregnant sharpnose sharks move offshore as a group during gestation and return to the shallows to give birth (Castro, 1983). Catches of spiny dogfish off New England, in which large adults are sought, result in catches composed primarily of females. In the western Australian fishery, gummy sharks also are found in single sex groups. Many fisheries operate at only certain times of the year or in selected locations and thus may have a propensity to target, intentionally or unintentionally, a certain sex or maturity stage. The Mexican artisanal shark fishery, for example, catches a large proportion of neonate and juvenile sharks in its inshore sets (Castillo-Geniz, 1998). Other fisheries target sharks in the same location at different times of the year, resulting in catches of seasonally different sexual maturity groups. The northwest Atlantic bottom longline fishery catches sexually mature sandbar sharks (Carcharhinus plumbeus) in the summer and immature sharks in the winter in North Carolina waters (Burgess and Johns, 1999). Sharks generally have a long gestation period, produce few young and reproduce on yearly, biannual or even triannual basis. Large fishing mortality on one sex or on a particular state of maturity can adversely affect the dynamics of a population. For that reason, it is imperative that representative samples of the sex and maturity composition of the catches are obtained regularly.
11.9.2 Identification of males and females
The sex of a shark is easily identifiable by the presence of claspers in males and their absence in females. Whenever possible, clasper size and maturity should be recorded for males and uterine condition, average ovum diameter and the sizes and sexes of embryos, for females,. These observations should be taken following the protocols described in Section 7.
11.9.3 Reproductive data collection
Recording reproductive data of female sharks is much more labor and time intensive. The ability to collect this data is dependent on the training of data collectors and time considerations. Section 7 provides a full discussion of determining maturity stages in female sharks.
11.10 AT SEA VERSUS SHORESIDE SAMPLING
11.10.1 Options for data collections
Several methods are used to collect fisheries and biological data. These include fisheries observers, shore and dockside sampling, logbooks and surveys. Each have positive and negative aspects and the decision to use any particular one usually depends upon the size of the vessels in the fishery, the length of fishing trips, which data are desired and the funding available to support data gathering. The data that are collected will only be as good as the method and people used in their collection. A combination of two or more methods is usually required for adequate data gathering.
11.10.2 Fisheries observers
Fisheries observer programmes are used worldwide to collect fisheries data including biological data, species composition, discards, etc. This is the preferred means of gaining accurate and in-depth data, but it is more costly than other data gathering methods. Observers should be trained in biology and should be able to obtain better quality data than fishers. Observers receive training in collection and sampling techniques from fishery professionals involved with, and often employed by, the fishery organization that manages the fishery. Observer programmes tend to be enacted after a fishery has demonstrated a decline, but their use is a wise monitoring strategy in well-managed or developing fisheries as well. The amount of data observers collect depends on the goals of the management organization. Observers can collect a variety of information, including fishing location and depth, time of gear sets and metrical, oceanographic data (e.g. water temperature and salinity), type and amount of gear used, species identification, catch vitality, sex, lengths and weights, maturity and biological samples (Figure 11.8). Observers are extremely beneficial to management programmes because of the amount and accuracy of the information they collect. However, such programmes can be expensive, time consuming and impractical if the boats in the fishery are too small.
11.10.3 Shore-side sampling
Shoreside and dockside sampling is useful in fisheries where sharks are landed whole, such as recreational and some artisinal fisheries. Unfortunately sharks often are dressed at sea and landed headed and gutted, which can pose significant problems for land-based sampling since species identification, sex, fork and total length, reproductive sampling and at-vessel vitality cannot be determined. If sharks are landed intact, then a shore-based data collector can record much of the same data as an at-sea observer. Elicited cooperation with fishing captains can lead to provision of additional data, such as fishing location, depth, type and amount of gear used, lengths of sets, etc. If the exact location is known, fishing charts can be used to determine the depth and water temperatures can be estimated in some circumstances using existing oceanographic data. There are several ways to conduct shore or dockside sampling.
The number of boats sampled is dependent on what percentage of the fishery each management organization is interested in observing. That percentage is most often determined by the fiscal constraints. An example of a data gathering form is shown in Figure 11.7.
Logbooks are used in many fisheries but logbook data maybe highly variable and can be suspect. Despite this, logbooks data are commonly used in stock assessments and one the major data collection source in numerous fisheries. Fishers are required to fill out logbooks while at sea. The following data can be recorded in logbooks: species identification, number caught, sex, size, disposition, gear type and amount used, gear modifications, location, time of set and haul back, depth and water temperature (Figure 11.8). It is widely recognized that fishers do not always record data about their catches accurately and frequently identify species incorrectly. Fishers busy bringing in and working up their catch are unlikely to record accurate data at the expense of fishing productivity. Many fishers do not fill in their data at the time of fishing and recreate data from memory at later dates. Fishers'illiteracy is a problem in some regions. Correct species identification is a major issue, because most fishermen are not scientifically trained in proper identification techniques. In addition, many fishers dislike any type of management planning and are unwilling to go out of their way to collect data. Finally, there may be no quality control of logbook data, with no on-board monitoring of logbook entry. However, this type of data collection is inexpensive and is often the only method available if funding is lacking or if vessels are too small to take observers. Some estimates of the accuracy of logbook data may be available when limited observer data are also available from the same fishery. This can be done by comparing the observers record of various parameters to those in the logbook records.
Examples of data sheets used by fisheries observers (National Marine Fisheries Service/FLMNH).
Examples of a shoreside survey data collection sheet (National Marine Fisheries Service).
Example of a logbook used to collect data by fishers while at sea (National Marine fisheries Service/FLMNH).
11.10.5 Telephone and dockside sampling
Telephone or dockside surveys used to monitor recreational fishers involve either calling, or going to the docks and interviewing fishermen about their trips as they come back in. Interviewers usually ask questions about the species targeted and catch composition, type and amount of gear employed, gear modifications and sizes and size of the vessel. This is a basic type of data collection and there are various problems associated with this type of sampling. Interviews are often done several days after a trip, which results in fisher memory lapses and poor data quality. But, as in logbook data, this type of data gathering is relatively inexpensive and provides an alternative to more costly methods.
The use of fishery dependent data is a vital component of the fishery management process. Section 11 provides the tools necessary for managers from different areas to determine what type of data should be collected and how to collect it. The methods used will vary depending on locality, experience and the types of management plans used. In all cases the collection of even the simplest data set may help eliminate the threat of overfishing and subsequent population collapses.
11.12 LITERATURE CITED
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Bonfil, R. 1997. Status of shark resources in the southern Gulf of Mexico and Caribbean: implications for management. Fisheries Research, 29: 101–117.
Burgess, G. & Johns, K. 1999. Commercial shark fishery observer program: analysis of the large coastal shark fishery-July and August 1998 season in the southeastern United States, with a review of the 1998 commercial shark fishery in the region. Final Report to Highly Migratory Species Division, National Marine Fisheries Service, Silver Spring, Maryland. 19 pp.
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