Steven J. D. Martell

Counterintuitive Responses of Fish Populations to Management Actions

Abstract Observed ecosystem responses to fisheries management experiments have often been either much smaller or in the opposite direction of the expected responses based on experience or population models. Examples of these responses can be found even for some very simple experimental management manipulations such as predator and prey manipulations in small lakes and ponds to fish population responses to harvest closures. Such counter-intuitive prediction failures offer opportunities to identify key processes and variables that are not widely considered in models used to evaluate ecosystem-based fisheries management policies. A common denominator in the case histories presented are unexpected behavioral responses and strong changes in juvenile survival rates of fish driven by changes in competition, predation, and behavioral responses to predation risk. These factors restructured many of the ecosystems in our simple examples, yet are not widely included in models currently used to evaluate ecosystem-base...

Parameterizing age-structured models from a fisheries management perspective

Age-structured models are widely used in fisheries stock assessments and contain two very important parameters that determine the rate and amount of harvest that can be safely taken: the compensation rate in juvenile survival (κ) and the unfished biomass (Bo). These two parameters are often confounded. It is common for relative abundance indices to lack contrast, and the use of informative priors, or fixing at least one of these parameters, is necessary to develop management advice. Providing management advice proceeds by transforming estimates of biological variables such as Bo and κ into management variables such as the maximum sustainable yield (C*) and the fishing mortality rate that would achieve this yield (F*). There is no analytical solution for the transformation of Bo, κ to C*, or F* for age-structured models with commonly used stock–recruitment functions and therefore must be done numerically. The opposite transition, however, does have an analytical solution for both the Beverton–Holt and Rick...

Age-Structured Mark-Recapture Analysis: A Virtual-Population-Analysis-Based Model for Analyzing Age-Structured Capture-Recapture Data

Abstract We present a new model to estimate capture probabilities, survival, abundance, and recruitment using traditional Jolly–Seber capture–recapture methods within a standard fisheries virtual population analysis framework. This approach compares the numbers of marked and unmarked fish at age captured in each year of sampling with predictions based on estimated vulnerabilities and abundance in a likelihood function. Recruitment to the earliest age at which fish can be tagged is estimated by using a virtual population analysis method to back-calculate the expected numbers of unmarked fish at risk of capture. By using information from both marked and unmarked animals in a standard fisheries age structure framework, this approach is well suited to the sparse data situations common in long-term capture–recapture programs with variable sampling effort.

Can Cooperative Management of Tuna Fisheries in the Western Pacific Solve the Growth Overfishing Problem

Tuna fisheries in the western and central Pacific Ocean are important globally for both food and economic security. Yellowfin and bigeye tuna stocks in this region are declining, in part due to the juvenile bycatch of these species by the purse seine fishery using floating objects and fish aggregating devices (FADs). This leads to a conflict between the longline and handline fisheries, which target adult fish, and the purse seine fishery, whose bycatch is leading to growth overfishing. This paper develops a bioeconomic game-theoretic equilibrium model to determine if, at equilibrium, the elimination of juvenile fishing could bring economic benefits to the region. Specifically, we examine non-cooperative and cooperative outcomes for a three-player game: purse seine, longline and handline, incorporating skipjack, yellowfin and bigeye as target species. Our results suggest that the reduction or elimination of fishing on floating objects could result in increased net benefits to the region from US

Experimental policies for rebuilding depleted stocks

100 million to US

Fisheries ecology and management

350 million per year; however, this gain is not shared equitably among the three fisheries. Notably, purse seine fisheries stand to lose economically through this type of management decision. The realization of potential economic gains for this region will require the formation of a cooperative sharing system.

Possible ecosystem impacts of applying MSY policies from single-species assessment

Experimental policies for rebuilding stocks through reduction in fishing mortality rate can be classified by severity of the reduction and duration of application of the reduced rate before moving to a long-term fishing rate goal. For stocks ranging from highly productive shrimp (Penaeidae) in the Gulf of Mexico to longer-lived, less productive Atlantic yellowfin (Thunnus albacares) tuna, we show that highest expected long-term fishery values are likely to come from relatively harsh experiments (F reduced to lowest value likely to be optimum for the long term) for which there is a commitment to maintain the experimental fishing mortality rate for several generations. Shorter experiments and less severe reductions in fishing rate lead to high probabilities of poor choices for postexperimental fishing rate, while much longer experiments do not produce substantially more information yet forego opportunities for higher harvests.