Abstract
Long-term fish survey monitoring programs use a variety of fishing gears to catch fish, and the resulting catches are the basis for status and trends reports on the condition of different fish stocks. These catches can also be part of the data used to set stock assessment models, which establish harvest regulations, and to fit population dynamics models, which are used to analyze population viability. However, most fishing gears are size-selective, and fish size — among other possible covariates, such as environmental conditions — affects the probability that a fish will be caught in the path the gear sweeps. Failing to properly account for selectivity can adversely affect the ability to interpret and use status and trends measures, stock-assessment models, and population-dynamics models. Our side-by-side gear comparison study evaluated the selectivity of multiple open-water trawl surveys that have provided decades worth of information on the imperiled fish species Delta Smelt (Hypomesus transpacificus). We used data from the study to estimate gear selectivity curves for multiple trawls using two methods. The first method examines the total number of fish-at-length caught across all gears, and does not directly use or estimate fish length distribution in the population. The second method examines the total number of fish caught by each gear separately, and explicitly estimates fish length distribution in the population. The results from the two methods were similar, and we found that one trawl was highly efficient at catching larger Delta Smelt. This is the first formal multi-gear evaluation of the relative selectivity of how well survey gear used to monitor Delta Smelt in the San Francisco Estuary selects fish by size, and we plan to incorporate the results into Delta Smelt population models.
Highlights
Fishing gear selectivity is defined as the probability that a fish will be captured by a particular fishing gear, assuming the fish is located where the gear is deployed, or, more generally, assuming the fish is available to the gear
Fall Midwater Trawl Survey (FMWT) and Chipps Island Midwater Trawl Survey (CMWT) had the highest levels of sampling effort in terms of volume of water sampled, they produced low catches compared to SKT
This study estimated length-dependent selectivity curves for trawls used to monitor Delta Smelt, but further investigation is needed into other factors that affect gear efficiency such as tow method
Summary
Fishing gear selectivity is defined as the probability that a fish will be captured by a particular fishing gear, assuming the fish is located where the gear is deployed, or, more generally, assuming the fish is available to the gear. Selectivity is used synonymously with retention rate or retention probability, because it represents the probability that the gear will retain a fish. This probability typically depends on fish size. Smaller fish can escape through large-sized mesh in gears such as trawls, seines, and gill nets; larger fish may be able to swim out of the path of a towed gear (gear avoidance behavior). The ability of a fish to escape a trawl cod end — either by swimming through the mesh volitionally or by being pushed through via water pressure — depends on the fish’s girth and length (MacLennan 1992; Wileman et al 1996; Herrmann et al 2009). Selectivity is often described as a function of fish length since length is an easyto-measure proxy for overall size (Millar and Fryer 1999)
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