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Status of large predatory fish stocks in the Pacific Ocean
23 Jan, 2007 05:49 pm
In December 2006, my colleagues John Hampton from the Secretariat of the Pacific Community, Pierre Kleiber from NOAA Fisheries, Mark Maunder from the Inter-American Tropical Tuna Commission, and I published an analysis of the state of the populations of large predators in the Pacific Ocean. Here, I summarize some of the main points of our analysis. Details can be found in the original article and its supporting online material.
Oceanic tuna fisheries uses four principal gear types, longline, purse seine, pole and line (or baitboat), and troll, and each gear type may be deployed in a variety of configurations to target particular species. In the Pacific, these gear types are deployed by fishing vessels from approximately 25 different countries and territories. Fisheries management agencies collect copious data of from these diverse fisheries. The data from each fishery is a window into a different aspect of the life history of the fish populations. Fish catch and fishing effort may provide clues to fish abundance in specific areas and depth strata. Size composition of the catch provides provides insight into the ages of the fish in the catch. In addition, tagging experiments are periodically conducted to understand movement patterns and estimate exploitation rates. Unfortunately, simply looking though one of these windows, for instance only considering catch and effort data from the Japanese longline fleet from the South Pacific, will produce a biased conclusion about the status of the fish stocks.
Fisheries scientists meet the challenge of combining views from these multiple windows using well known principals of demographic population dynamics and modern statistical methods in integrated stock assessment models. Current state-of-the-art stock assessment models represent the population with an age-structured demographic model, describe dependence of catch on age of fish, include temporal variability in recruitment of young individuals into the population, and explicitly describe movement within the range of the stock. These models reconstruct the trends in biomass and size composition of the fish stocks during the period covered by the data. Fishing can be “turned off” in the models to estimate what the condition of the stock might have been in the absence of fishing given the observed variability in the environment. Thus, the “exploited biomass” is defined as the biomass estimated from the observed fishing history and the “unexploited biomass” as the biomass estimated in the absence of fishing. The ratio of the exploited to unexploited biomass is interpreted as a measure of the impact of fishing.
We used the MULTIFAN-CL integrated stock assessment model to analyze all available data from Pacific tuna fisheries for eight stocks of large predators: bigeye tuna (Pacific wide), yellowfin (eastern and western Pacific), skipjack (eastern and western Pacific), albacore (north and south Pacific) and north Pacific blue shark (Prionice glauca) for the period 1950-2004.
As might be expected, the estimated trends in biomass vary substantially among stocks. When compared to the biomass in 1950, western Pacific yellowfin and bigeye have declined steadily to levels near the equilibrium biomass that would produce the maximum sustainable yield (MSY) in the fishery. Eastern Pacific yellowfin, skipjack tuna and blue shark appear to have increased slightly, while albacore have fluctuated in both directions. This apparently conflicting appraisal of the impact of fishing illustrates the problems of using a fixed baseline to gauge impact. Expressing impact of fishing as the ratio of exploited to unexploited biomass includes the effects of environmental variability and may be a more suitable indicator than comparison of current biomass to the biomass at some arbitrary date in the past when the stock is assumed to be in a “pristine” or “virgin” state. The impact of the fishery is detectable in all stocks using the exploited to unexploited ratio.
Current biomass ranges among species from 36% (Western Pacific Yellowfin) to 91% (North Pacific Blue Shark) of the biomass predicted in the absence of fishing, a level consistent with or higher than standard fisheries management targets, such as MSY. The impact on the segment of the population that is of reproductive age has been more severe. The adult biomass ranges among species from 12% (North Pacific Albacore) to 89% (South Pacific Albacore) of that predicted in the absence of fishing.
Articles in high-profile scientific journals claim to show catastrophic declines in population sizes and warn of wholesale collapse of oceanic food chains. These prophecies are based on small, biased subsets of the data and on faulty analyses. Our results, using state of the art methods applied to all available data, indicate little impact of fisheries on some stocks but substantial, though not catastrophic, impacts on others. Bigeye, yellowfin and northern albacore are being harvested at rates that are considered unsustainable by many fisheries management agencies, and the international commissions responsible for managing these fisheries need to take action soon, before stocks decline to a point where fisheries become uneconomical and recovery of the stocks may be slow.
Effective management of marine resources will become increasingly difficult as the world's population of humans approaches 9 billion, standards of living increase, and the climate warms. Management solutions will require sound scientific analysis based on all available data. Exaggerated claims of collapse of marine fish populations and of oceanic ecosystems do not provide practical guidance for the future. Fisheries managers know what steps are required to conserve fish stock. The only thing missing is the will to implement the necessary restrictions on fishing.
Sibert, J., Hampton, J., Kleiber, P., and Maunder, M. 2006. Biomass, size, and trophic status of top predators in the Pacific Ocean. Science 314: 1773-76.
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