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2. Problems in understanding and assessing the status of shark populations

We encountered several problems in determining the impact of trade upon species of sharks. First, there is a general lack of biological knowledge about sharks. Second, we encountered a general lack of species-specific catch and effort statistics in the shark fisheries and in the shark bycatch. Third, the specific data needed for demographic and stock assessment models is simply lacking and most of the models used in stock assessment were developed for bony fishes and their application to sharks may be questionable. Below are the explanations for these generalized problems.

Lack of biological data: There are several reasons for the scarcity of basic biological knowledge on sharks and elasmobranchs, in general. In the past, there was little economic incentive or impetus to study sharks. The low value of shark meat was greatly exceeded by that of many teleosts (e.g., tunas, mackerels, snappers, groupers, etc.), thus the impetus was on elucidating the biology of valuable teleosts. Furthermore, prior to the emergence of the commercial shark fisheries of the middle 1980s, most students were discouraged from studying sharks by the logistical problems encountered trying to obtain specimens. As a result, most ichthyologists and fishery scientists studied teleosts and ignored sharks. Consequently, only a handful of the species of sharks involved in intensive fisheries in developed countries were studied. This situation has been changing in the last decade, because the increase in value of shark products and the ecological concerns about sharks. However, due to scarce research money and logistical problems, progress in shark biology continues to be slow.

Lack of fisheries data: Lack of fishery data on shark fisheries is critical to their proper assessment. Obtaining data is difficult, for the following reasons:

· Most fisheries simply do not report shark landings by species. They usually lump together all shark species and often all elasmobranchs are included together. The reasons for this practice are the lack of trained personnel capable of discriminating among species, and the lack of incentive to produce shark statistics. In most cases, we do not have catch or landings statistics by species, or by higher taxonomic level, to quantify fishing mortality of a given species of shark or species complex. The interpretation of the available generic data is difficult, since the catch species composition is not known. Even when the present catch composition is known, and good time series are available, one can not extrapolate the composition of past generalized landings, because one can not assume that the catch species composition has remained unchanged through time.

· Effort data are usually missing in fishery statistics, making the interpretation of landings statistics difficult.

· In the few cases where fisheries do have good statistics, they are often reluctant to publish such data, because they fear that restrictions will be placed on their fishing activities.

· The migratory patterns of some targeted sharks species complicate the analysis of fisheries data. They are caught by various fisheries in two or more countries, making it difficult to determine the total catch or the age structure of the total catch.

It is unlikely that the present lack of fishery data will change in the near future, because many countries simply lack the resources and infrastructure to monitor their fisheries adequately. It will take a concerted effort by interested countries and international agencies to improve the training of fishery workers in shark or elasmobranch identification before meaningful statistics on shark catches and landings can be expected.

Lack of suitable models: We lack the suitable population models to assess the impact of fishing and trade on sharks, and we do not know the sizes of shark populations or stocks. Most of the theoretical stock assessment models are based on bony fishes with life histories that are quite different from sharks. Consequently, attempts at shark stock assessment have been few and the results have been questionable or severely flawed. In recent years, demographic analysis has been used to analyze the effects of fishing upon shark populations. Only a handful of species have been analyzed (e.g., Cortés 1995, Sminkey and Musick 1995) and in most cases, the parameters necessary for analysis, such as age at maturity, mortality, etc., are not known.

Lack of validated age estimates: The growth rate of a species and its estimated age at sexual maturity are essential for stock assessment and demographic models. Information on growth of elasmobranchs has been derived from counts of opaque and translucent bands ("rings") in their spines and vertebrae, because elasmobranchs lack the hard parts (scales, otoliths, or bones) commonly used in age and growth studies of teleosts (Cailliet et al. 1986). Although, the mechanism of calcium deposition in these bands is not understood, the periodicity of these bands is usually taken as an indicator of annual growth. However, in some cases, such as in the basking shark, two such bands appear to be deposited each year (Parker and Stott 1965), and in the case of the angel shark, band deposition has been shown to be more closely related to somatic growth than to age (Natanson and Cailliet 1990). A few elasmobranch studies have evaluated the temporal periodicity of the band deposition in the spines and vertebral centra (Beamish and McFarlane 1983, Cailliet et al. 1986), and ever fewer have validated the age estimates. To our knowledge, no author has validated different age groups of any elasmobranch species, thus assuming that the rate of deposition is the same before and after maturity, when the growth rate changes abruptly. The extrapolation beyond the maximum age validated, between species, or between populations of the same species is, as Beamish and McFarlane (1983) stated, "dangerous".

Because of the limitations outlined above, the assessment of the status of shark species can be made on historical data (catches, landings, CPUE, etc.) in only a few cases. In the cases where fishery data do not exist, the status can be inferred from the unique characteristics of sharks, the reproductive potential of the species, growth coefficients, and from the level of exploitation being applied. These factors are explained below.

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