A historical review of selectivity approaches and retrospective patterns in the Pacific halibut stock assessment
Introduction
Integrated statistical fisheries stock assessments are now standard approaches in many parts of the world (Hilborn and Walters, 1992, Quinn and Deriso, 1999, Maunder and Punt, 2013, Fournier and Archibald, 1982, Megrey, 1989). These models fit to available fisheries-dependent and/or fisheries independent data and provide estimates of management-related quantities including reference points, stock size, and harvest rates. Time-series of catch and relative abundance estimates, together with biological information (lengths, ages, or both) from these time-series provide information on the population trend and the demographic components contributing to that trend.
A crucial aspect of these analyses lies in defining the observed number of fish at a particular length or age, relative to the number of fish estimated to exist at that length or age in the population dynamics model. This relationship is variously referred to as efficiency, effectiveness, or the combination of selectivity, and catchability. Because various definitions are used interchangeably in the fisheries literature, in this paper we identify and use four distinct terms:
- (1)
Availability: the relative probability a fish will be in the same area at the same time that the gear is being deployed.
- (2)
Vulnerability: the relative probability a fish that is present when and where survey (or fishing) gear is deployed will be captured (also commonly denoted as āgear selectivityā or ācontact selectivity).
- (3)
Selectivity: the length- or age-based probabilities used to relate fish predicted to exist in a population to those that are observed in the data; this represents the combination of both vulnerability and availability.
- (4)
Catchability: the scaling coefficient between an index of abundance (or catch-per-effort) and the abundance at length or age that is most selected.
There are a number of biological and technical factors that can contribute to differences in vulnerability, availability, or both as a function of fish length, age, or both (Olsen and Laevastu, 1983, provide a detailed conceptual map of many factors influencing longline catch rates in general). Biological factors can include ontogenetic shifts among habitats, behavioral differences due to changes in diet, differences in growth rates among different habitats, morphology (jaw dimension, body length or shape, etc.), and many others. Technical factors may include physical aspects of sampling/fishing (mesh or hook size, set duration, towing speed, etc.), gear performance in different habitats, regulatory length-limits, and many others. These factors may be temporally variable or static; however in both cases interactions among them may result in highly variable selectivity or catchability over time.
The Pacific halibut stock assessment and management system has a long history of data collection and scientific analysis, serving as a testing ground for many of the fisheries modeling approaches that have been developed over the last several decades (Clark, 2003). Despite this history (or perhaps causing this history), Pacific halibut present a suite of difficult challenges to the modeling of selectivity. Such challenges are frequently present in other fisheries, but are infrequently observed en masse in a single stock assessment application. As such, Pacific halibut represent a unique and potentially illustrative case-study.
In this manuscript, we review the approaches taken over several decades of the Pacific halibut stock assessment, with a particular emphasis on the treatment of selectivity. We identify a recurrent theme of simplifying selectivity assumptions that, over three distinct time-periods, each became increasingly mismatched with the underlying population dynamics. We summarize the retrospective patterns in biomass estimates (and therefore management-related quantities) that appear to be a result of these mismatches. We then present the results of the most recent stock assessment as a more flexible long-term solution to these historical issues.
Section snippets
A brief overview
The Pacific halibut stock assessment (Stewart et al., 2013a), conducted annually by the International Pacific Halibut Commission (IPHC), estimates the status of the resource in the northeastern Pacific, including the territorial waters of the United States and Canada (Fig. 1). The directed halibut fishery, closely monitored and managed for nearly 100 years, is prosecuted primarily with longline gear throughout its geographic range (Gilroy et al., 2013). Other sources of removals include sport (
Biological and technical factors
The Pacific halibut fishery and survey exhibit a large suite of factors potentially contributing to selectivity. These factors are both biological and technological in nature, with a strong potential for among-factor interactions.
The most important selectivity-relevant biological factor for Pacific halibut may be the dramatic changes in length-at-age observed over the historical record. Halibut exhibit highly sexually dimorphic growth, with females reaching much larger sizes than males (Martell
Historical stock assessment approaches
The Pacific halibut resource has been analyzed by many widely known fisheries scientists using a variety of analytical tools (Clark, 2003), and a stock assessment has been performed annually since the late 1970s (Table 1). Early stock assessments focused on equilibrium catch rates and sustainable yield available from the stock (Thompson and Bell, 1934, Chapman et al., 1962). These analyses represent the origin of the debate regarding the relative effects of fishing vs. environment factors (
Evolution of a more general approach to selectivity
In 2012, based on the potential management implications of the recent retrospective pattern (Valero, 2012) and starting from the detailed investigations during recent assessment processes, a thorough investigation of the assessment model software and behavior was conducted (Stewart et al., 2013b). Despite careful inspection, no significant coding errors or inconsistencies in data preparation that appeared to be contributing to the retrospective bias were discovered. Treatment of bycatch
Discussion
There is an expanding application of time-varying catchability and selectivity in stock assessment analyses reflecting the growing recognition of the many changing factors observed for Pacific halibut. In our case, these factors were found to have pronounced implications for assessment results and management decisions. The recurrent mismatch between, and need for revision of, simple assumptions and changing dynamics has led to several difficult paradigm shifts in the halibut stock assessment.
Acknowledgements
We thank the many scientists who have analyzed the available historical data for Pacific halibut over the history of the Commission. We particularly thank Bill Clark, Steven Hare, Bruce Leaman, and Juan Valero for extensive evaluation of model behavior, retrospective patterns, and exploration of potential contributing factors prior to the 2012 stock assessment. Jim Ianelli and Robyn Forrest provided valuable comment and insight into the recent investigations into the stock assessment model. The
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