A Covered Cod-End and Tow-Path Evaluation of Midwater Trawl Gear Efficiency for Catching Delta Smelt (Hypomesus transpacificus)

Lara Mitchell,Randall Baxter,Ken Newman

doi:10.15447/sfews.2017v15iss4art3

Abstract

Author(s): Mitchell, Lara; Newman, Ken; Baxter, Randall | Abstract: For nearly 50 years, the California Department of Fish and Wildlife has used a midwater trawl to intensively monitor fish populations in the San Francisco Estuary during the fall, sampling over 100 locations each month. The data collected have been useful for calculating indices of fish abundance and for detecting and documenting the decline of the endangered fish species delta smelt (Hypomesus transpacificus). However, efforts to calculate estimates of absolute abundance have been hampered by the lack of information on gear efficiency, in particular questions about contact selectivity and the effect of tow method on catches. To answer these questions we conducted a study that used a covered cod end on a net towed either near the surface, referred to as a surface tow, or throughout the water column, referred to as an oblique tow. A contact selectivity model was fit to estimate the probability that a delta smelt that has come into contact with the net is retained in the cod end of the net conditional on its body length. Full retention of delta smelt was found to occur around 60 mm fork length. Delta smelt catch densities for the surface tows were an order of magnitude greater than densities in the oblique tows, suggesting a surface orientation at the sub-adult life stage. These results represent an important step in being able to calculate absolute abundance estimates of the delta smelt population size using decades’ worth of monitoring data.

Highlights

Long-term fish monitoring surveys such as the Fall Midwater Trawl (FMWT) Survey provide information on temporal changes in the relative abundance and spatial distribution of many fish species in the San Francisco Estuary (Feyrer et al 2007; Sommer et al 2007a; Sommer and Mejia 2013)
Absolute abundance estimates from different surveys at different times of year, targeting different life stages, could be integrated into analyses designed to assess the effects of management actions and environmental conditions on population dynamics, and, in the case of endangered or threatened species, to assess population recovery (Somerton et al 1999; Methot and Wetzel 2013)
We address objective (2) by comparing Delta Smelt catch densities from surface tows, where the net is towed in the upper portion of the water column, and oblique tows, where the net is dropped near the bottom and brought up to the surface along an inclined path, and by discussing potential causes for observed differences in density that relate to fish behavior and sampling procedures

Summary

Introduction

Long-term fish monitoring surveys such as the Fall Midwater Trawl (FMWT) Survey provide information on temporal changes in the relative abundance and spatial distribution of many fish species in the San Francisco Estuary (Feyrer et al 2007; Sommer et al 2007a; Sommer and Mejia 2013). Though indices of abundance are informative and relatively simple to calculate, being able to generate estimates of absolute abundance for fish species of interest in the estuary is desirable. Absolute abundance estimates from different surveys at different times of year, targeting different life stages, could be integrated into analyses designed to assess the effects of management actions and environmental conditions on population dynamics, and, in the case of endangered or threatened species, to assess population recovery (Somerton et al 1999; Methot and Wetzel 2013). In the case of Delta Smelt, our underlying goal is to use abundance-based models to help answer increasingly important management questions about the current population size and the proportion of the population that is killed by water export operations in the estuary (Newman 2008)

Objectives

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Discussion

Conclusion