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11. Fishery-Dependent Sampling: Total Catch, Effort and Catch Composition

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11. Fishery-Dependent Sampling: Total Catch, Effort and Catch Composition 182 11. Fishery-dependent sampling: total catch, effort and catch composition Alexia C. Morgan and George H. Burgess Florida Museum of Natural History Division of Fishes University of Florida Gainsville, Florida <[email protected]> 11.1 INTRODUCTION 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 11. Fishery-dependent sampling: total catch, effort and catch composition 183 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. 11.2.3 Bycatch 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 184 Management techniques for elasmobranch fisheries 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 11.3.2.1 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 FIGURE 11.1 fisheries managers are seeking from Three variations in the placement and design of gillnet CPUE data dictates the catch and unit fishing gear. The net floats and anchors are all visible effort measures used to calculate CPUE. (NOAA). The following are examples of possible CPUE calculations: 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 11. Fishery-dependent sampling: total catch, effort and catch composition 185 this case, the CPUE’s numerator and denominator are often multiplied by an exponent of 10 (e.g. 1 000) 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 1 000 panel-hours.
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