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

Lara Mitchell, Ken Newman, Randall Baxter

Research output: Contribution to journalArticlepeer-review


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 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
Original languageEnglish
Article number17(1)
Number of pages24
JournalSan Francisco Estuary and Watershed Science
Issue number1
Publication statusPublished - 18 Mar 2019


  • size selectivity, fishing gear


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