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IJMS Editor's Choice - Reef fish discard mortality

The latest Editor’s Choice article from ICES Journal of Marine Science is now available. Here, researchers look into discard mortality for red snapper and gray triggerfish, two valuable fish stocks in the US Gulf of Mexico.
Published: 8 November 2019

​​Red snapper (Lutjanus campechanus) are perhaps the most targeted and culturally iconic fish species in the northern Gulf of Mexico. After many decades of heavy exploitation, the red snapper stock in the US Gulf of Mexico was depleted to historically low levels by the 1980s and remains under a rebuilding plan. The recreational fishery is managed with daily bag limits, minimum fish length requirements, and seasonal fishery closures. 

Gray triggerfish (Balistes capriscus), also a popular species among recreational anglers, commonly co-occur with red snapper. Gray triggerfish have experienced overfishing in recent years and are now subjected to more restrictive management, including seasonal fishery closures. 

One consequence of these regulations is that recreational anglers discard the majority of red snapper and gray triggerfish they catch. Discard mortality estimates vary for both species, but 11.8% and 5% were assumed in the most recent stock assessments for red snapper and gray triggerfish, respectively. While large numbers of estimated dead discards result for each species assuming those discard mortality rates, concern exists that discard mortality may exceed those values, which would represent even greater sources of wasted removals from these valuable fish stocks.

Does rapid recompression reduce discard mortality?

In the latest Editor's Choice paper​, researchers from the University of Florida used an advanced geopositioning acoustic telemetry array to track the fine-scale movements and fates of tagged red snapper and gray triggerfish for up to 1 year over a 15 km2 study area in the northern Gulf of Mexico. The primary objective was to evaluate whether rapid recompression (releasing fish with weighted return-to-depth tools, also known as descender devices) reduced discard mortality. Descender devices include several different designs of weighted hooks, clamps, and cages that have been used in other US fisheries, most notably recreational rockfishes off California, Oregon, and Washington. 

Large bull sharks (Carcharhinus leucas) tagged off the east coast of Florida by researchers from University of Miami Rosenstiel School of Marine and Atmospheric Sciences and Bimini Biological Field Station Foundation in the Bahamas were also detected and tracked within the acoustic array, yielding information on movement patterns of these large predators. Many tagged red snapper and gray triggerfish displayed movement patterns similar to tagged bull sharks almost immediately following release, suggesting those fish were consumed by predators. 

Overall, discard mortality estimates for red snapper (36.1–56.6%) and gray triggerfish (26.7–60.0%) greatly exceeded assumed values in recent stock assessments, while predation by large pelagic predators was the dominant source of discard mortality. Discard mortality due to predation has likely been overlooked in previous mark-recapture, laboratory, and enclosure studies, suggesting cryptic population losses due to predation on discards may be underestimated for Gulf of Mexico reef fishes. Descender devices were effective in reducing discard mortality of red snapper by approximately half, which would constitute a major conservation benefit if descender devices were widely used.

R​​ead the full paper​ on ICES Journal of Marine Science.

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A digital image captured by an upward facing GoPro video camera showing a large shark (species unknown) immediately before consuming an acoustic telemetry tagged red snapper (45 cm TL) released from a descender device. The silhouettes of both fish are visible against the light of the sky and the SeaQualizer descender device is in the center of the image. Photo: Erin C. Bohaboy.

​Paper:
Application of three-dimensional acoustic telemetry to assess the effects of rapid recompression on reef fish discard mortality

Authors:

Erin Collings Bohaboy, Tristan Guttridge,
Neil Hammerschlag, Maurits P.M. Van Zinnicq Bergmann, William F. Patterson III.

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IJMS Editor's Choice - Reef fish discard mortality

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