Estimating the Size Selectivity of Fishing Trawls for a Short-Lived Fish Species

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.

Using Multistage Design‐Based Methods to Construct Abundance Indices and Uncertainty Measures for Delta Smelt

“…Another caveat is that the estimated length‐based contact selectivity functions, the (Mitchell et al. , ), may be biased and inadequate. Skepticism about the ascending and descending limbs of dome‐shaped selectivity curves led to sensitivity analysis using the truncated curves and the effects on the resulting abundance indices were sizable, e.g., up to a 10‐fold decrease from nontruncated to truncated estimates.…”

“…This was of particular concern for the non‐monotonic 20‐mm and STN selectivity curves identified by the data, which were not informed by many captures of large fish (Mitchell et al. ). To investigate the effects of this problem, we compared indices based on the original selectivity curves with estimates based on “truncated” curves, defined to be the same as the original curves except with the descending tail of each curve replaced by a horizontal line at 1.0 (see Figure 7 in Mitchell et al.…”

“…Length‐based, gear‐specific contact selectivity functions were obtained from Mitchell et al. (, ). In Mitchell et al.…”

“…In Mitchell et al. (), different combinations of 20‐mm, STN, and SKT gear were deployed more or less simultaneously in the same area. In this case, because direct information on the length distribution of the population is not available, the estimated curves are relative selectivity curves (Millar and Fryer ).…”

“…To investigate the effects of this problem, we compared indices based on the original selectivity curves with estimates based on “truncated” curves, defined to be the same as the original curves except with the descending tail of each curve replaced by a horizontal line at 1.0 (see Figure 7 in Mitchell et al. ).…”