Abstract
Resource managers often rely on long-term monitoring surveys to detect trends in biological data. However, no survey gear is 100% efficient, and many sources of bias can be responsible for detecting or not detecting biological trends. The SmeltCam is an imaging apparatus developed as a potential sampling alternative to long-term trawling gear surveys within the San Francisco Estuary, California, to reduce handling stress on sensitive species like the Delta Smelt (Hypomesus transpacificus). Although believed to be a reliable alternative to closed cod-end trawling surveys, no formal test of sampling efficiency has been implemented using the SmeltCam. We used a paired deployment of the SmeltCam and a conventional closed cod-end trawl within the Napa River and San Pablo Bay, a Bayesian binomial N-mixture model, and data simulations to determine the sampling efficiency of both deployed gear types to capture a Delta Smelt surrogate (Northern Anchovy, Engraulis mordax) and to test potential bias in our modeling framework. We found that retention efficiency—a component of detection efficiency that estimates the probability a fish is retained by the gear, conditional on gear contact—was slightly higher using the SmeltCam (mean = 0.58) than the conventional trawl (mean = 0.47, Probability SmeltCam retention efficiency > trawl retention efficiency = 94%). We also found turbidity did not affect the SmeltCam’s retention efficiency, although total fish density during an individual tow improved the trawl’s retention efficiency. Simulations also showed the binomial model was accurate when model assumptions were met. Collectively, our results suggest the SmeltCam to be a reliable alternative to sampling with conventional trawling gear, but future tests are needed to confirm whether the SmeltCam is as reliable when applied to taxa other than Northern Anchovy over a greater range of conditions.
Highlights
Most sampling methods in fisheries and wildlife are imperfect (Royle et al 2005)
Surveys were conducted using the same equipment and operating protocols as normally deployed by the California Department of Fish and Wildlife (CDFW) during Fall Midwater Trawl (FMWT) surveys (Feyrer et al 2007, 2013; https://www.dfg.ca.gov/delta/data), and near fixed sampling stations sampled by CDFW monitoring programs (Napa River stations 340 and 341; San Pablo Bay stations 310 and 311; https://www.dfg.ca.gov/delta/data/fmwt/stations. asp)
The highest number of Northern Anchovy captured with either gear type occurred in San Pablo Bay, where mean observed counts were approximately 8 to 12 times higher than counts from the Napa River (Table 2)
Summary
Most sampling methods in fisheries and wildlife are imperfect (Royle et al 2005). Not detecting an individual during a survey does not VOLUME 19, ISSUE 2, ARTICLE 6 exclusively mean a species is absent, but may reflect whether the species was available to or retained by the chosen gear Such imperfections in the observation process propagate into the desired latent state metric (e.g., abundance, survival, diversity) that a study is designed to estimate. Designing experiments where closure is certain is relatively simple when surveys occur within small water bodies (e.g., using block nets within wadeable streams); it is difficult to ensure demographic closure in larger water bodies (large rivers, lakes, estuaries), and creative solutions are required This difficulty is exacerbated for species with conservation status, especially if the most efficient gear types coincide with adverse effects (e.g., greater fish mortality when electrofishing or trawling)
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