The Spot Prawn Fishery: A Status Report - Earth Economics

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The Spot Prawn Fishery: A Status Report Once the larvae begin to settle out, they migrate to inshore habitats suitable for growth and maturation, where they are believed to enter a relatively sedentary stage. While adult prawns have been known to move up and down in the water column in a diel (24-hour) migration pattern, it is not known if extended lateral migrations to adjoining coastal areas occur. Unpublished tagging studies carried out by J.A. Boutillier of Fisheries and Oceans Canada suggest that mature animals Egg-Bearing Female may well apply to prawns because of their lengthy pelagic larval stage” (Boutillier and Bond 1999a). Studies looking at smooth pink shrimp off the west coast of Vancouver Island have shown sequential recruitment among the population. This supports the concept of metapopulation trends in the region. Catch sampling data have illustrated good recruitment of a single spot prawn year-class over a fairly large area (Boutillier unpublished data, cited in Boutillier and Bond 1999a). It will be challenging to understand these processes and the factors and relationships affecting them. The abundance of egg-bearing females, the timing and length of spawning, the rates of development and growth, and larval production and survival are all influenced and controlled by water temperatures. In any given year, larval survival is affected by climatic conditions; for example, prevailing ocean currents. These can contain larvae in certain areas and possibly expose them to differing concentrations of planktonic foods, thus affecting growth rates in early life stages. In addition, the amount of quality habitat available to settling juveniles is likely to play a significant role in overall survival and population abundance (David Love, ADFG. Pers. comm., February 2001). Photo Courtesy K.M. Kattilakoski, Fisheries and Oceans Canada “remained within one mile or two of their release location over a period of several months” (Boutillier and Bond 1999a). Other indicators, such as parasite loads and growth rates, vary considerably between prawn stocks separated by even tens of kilometers. These support the view that, once the animals settle, little movement occurs (Bower and Boutillier 1990, Bower et al. 1996). Log books and catch samples suggest that a single year-class could settle in a particular area, live out its life cycle, and leave the area “virtually barren” when the year class dies off (Boutillier and Bond 1999a). If this is in fact the case, it is likely that there are hundreds of independent, localized adult stocks throughout the spot prawn’s geographic range. However, “the concept of meta-populations 3 (mixed common populations) that share larvae Predator-Prey Relationships Spot prawns are opportunistic foragers that typically feed on other shrimp, plankton, small mollusks, worms, sponges, and dead animal material. Adults are believed to be benthic (bottom) feeders that forage mostly at night. Spot prawns in turn are prey for other pelagic and demersal marine predators. As has been reported in other parts of the world for Pandalus borealis eous, or the pink shrimp, predators can play an important role in determining the reproductive success and recruitment of spot prawns to the fishery. To date, such studies have not been conducted in the spot prawn’s geographic range. Mortality due to predation is likely to be quite high during the larval and juvenile stages, but is significantly reduced once the animals settle out of the water column (Fisheries and Oceans 2000a). In benthic habitats, spot prawns __________________________________________________________ 3 The term metapopulation was first defined by Levins (1969) as “a population of populations” that occupies a certain percentage of the suitable habitat available (Vandermeer and Carvajal 2001). Levins (1969) theorized that increases in local extinction rates or reductions in colonization rates threaten the long-term viability of a given metapopulation. Numerous studies have provided support for these ideas, illustrating that local extinction rates increase with a decrease in the size of habitat patches, and colonization rates decrease as distances between habitat patches increase (Vandermeer and Carvajal 2001). 5
The Spot Prawn Fishery: A Status Report are prey for bottom-dwelling fish and octopus. Mid-water fish species such as salmon are not known to prey on spot prawns. The abundance of predator species may have an effect on the abundance of spot prawns and other shrimp species. A study in the Barents Sea demonstrated a significant negative correlation between the abundances of cod and northern pink shrimp (Berenboim et al. 1996). Several other studies provide evidence that where fishing pressure has reduced predator populations, prey populations have increased (Witman and Sebens 1992; Aronson 1989). Conversely, it is possible that the removal of shrimp by a commercial fishery plays a role in reducing the population of predator species (Fisheries and Oceans 2000b). Factors Affecting Spot Prawn Success Spot prawn reproductive and recruitment success is dependent on and likely to be affected by a broad range of environmental variables, ecological factors, and changes in ambient conditions. It is important to consider environmental parameters in the development of management systems. The following elements are likely to affect the “success” of spot prawn reproduction and recruitment (ADFG 1985): •variation in preferred water temperatures, pH levels, dissolved oxygen concentrations, and/or the general chemical composition of the water •modification of critical benthic habitat •alterations of intertidal areas •increases of suspended organic or mineral material •reduced food supply •reduced protective cover; e.g., seaweed beds •obstruction of migratory pathways •level of harvest AN OVERVIEW OF SPOT PRAWN MANAGEMENT The spot prawn fishery spans an enormous and diverse stretch of ecosystems and management jurisdictions. While there are inherent similarities in both the ecological and management systems throughout the animal’s range, there are numerous differences. These similarities and distinctions are enumerated and discussed in detail in the later sections of the Report. The table on p.7 is an effort to summarize the nature of the fishery and its management in each of the five jurisdictions, and to set the stage for the more detailed discussions that follow. ALASKA SPOT PRAWN FISHERY Biological Status of Spot Prawns Southeastern Alaska historically has been described as the “shrimp treasure house” (Brian Paust, University of Alaska Marine Advisory Program. Pers. comm., June 2001). While other regions—such as Prince William Sound, Kachemak Bay, and the waters off the coast of Kodiak Island— once supported spot prawn populations large enough to sustain a commercial harvest, this is no longer the case. Southeastern Alaska is now the locus of spot prawn commercial activity. (Please note: For the purposes of the Status Report, the “Alaska” section will focus primarily on the southeastern Alaska spot prawn fishery.) Spot prawns remain “cryptic” organisms whose long-term sustainability and appropriate harvest are challenged by this lack of basic biological information (Paust, University of Alaska Marine Advisory Program. Pers. comm., June 2001). At this stage, data suggest that many different areas and subpopulations of a greater metapopulation exist in the region (Love, ADFG. Pers. comm., February 2001). A stated goal of management is to further this biological understanding of spot prawn abundance and distribution. Analysis of preliminary research suggests that spot prawns in southeastern Alaska may be vulnerable to serial depletion. However, results are still under review, and the data are inconclusive. Whether serial depletion is due to changing environmental conditions or the effects of fishing is not currently known (see Piatt and Anderson 1996 in Orensanz et al. 1998). In the 1960s and ’70s, the Alaska Department of Fish and Game (ADFG) collected limited catchdistribution and pot-efficiency data. A 1996 review of catch-per-unit-effort (CPUE) data available through fish tickets led to a recognition that the amount and type of biological data available were inadequate for effective spot prawn management. A stock assessment protocol to gather more information was developed, and a multi-year pilot study to obtain CPUE, size and weight, and size and sex data was begun in Fishery Management Districts 3 and 7 prior to the 1996–97 fishery. The goal of this study was to: “collect and evaluate data required for rational management, to understand the variability of various parameters associated with stock assessment, to investigate factors essential to establishing an appropriate stock assessment program, and to provide information necessary to develop a well 6
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