Shrimp bycatch in the United States Gulf of Mexico: Review and evaluation of bycatch effects on exploited fish stocks

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Marine Biology and Fisheries

First Committee Member

Nelson M. Ehrhardt - Committee Chair


Finfish bycatch by the US Gulf of Mexico shrimp fishery is an important issue in the management of fisheries resources given the perceived high bycatch mortality on different exploited fish stock in the region. Data from the Bycatch Characterization Project was used in a simulation modeling approach to determine the effects of shrimp bycatch on stock reproductive dynamics and reduction of potential yields for Spanish mackerel ( Scomberomorus maculatus), king mackerel (S. cavalla), red drum (Sciaenops ocellata), and red snapper ( Lutjanus campechanus) in the US Gulf of Mexico. A simulation model (ByFish) was developed that couples the biological dynamics of the fish stocks, commercial and recreational directed fisheries, and the distribution of shrimp fishing effort in a spatio-temporal matrix.An evaluation of the current protocols to generate bycatch estimates revealed that bycatch data does not conform to the general linear model (GLM) assumptions. Instead a delta lognormal model appears as a better alternative. Annual bycatch estimates differed from the two models, in particular for Spanish and king mackerel, and red snapper.ByFish simulations show that bycatch reduced the spawning potential of the fish stocks, effect that increases with higher levels of exploitation from the directed fisheries. At exploitation levels of overfishing definition (OD) levels, for Spanish mackerel and red snapper potential ratios (SPR) declined 18% and 11%, respectively due to shrimp bycatch compared to SPR levels at F rate of maximum sustainable yields (MSY). In terms of yields for the directed fisheries, MSYs were reduced 40% and 50% for Spanish and king mackerel, respectively, and 51% for red snapper due to bycatch.Under the simulation conditions and with shrimp bycatch, biological reference point (BRP) targets of F0.1 was the most conservative exploitation rate in terms of spawning potential (SPR), while FMAX and F 50%Rmax drove the stocks of Spanish mackerel and red snapper into overfished conditions. The red drum stocks were the least affected of all stocks. The ByFish simulator was also used to evaluate different alternatives for bycatch reduction. Gear selectivity modifications (BRD) were more effective in reducing shrimp bycatch compared to shrimp fishing effort restrictions. Experiments show that only in the case of a BRD design with 50% selectivity reduction was effective to reduce F bycatch rates above the 44% target specified for red snapper. Spatio-temporal analyses of bycatch by species and shrimp effort distribution show that area 3 and 4 (off Texas and Louisiana coast) and seasons 2 and 3 (May--December) were the highest season-area (SA) grids of bycatch. Overall these SA account for more than 75% of total bycatch. Experiments that target these particular areas with BRD implementation and shrimp fishing effort restrictions show reductions of F bycatch of 48% and 54% for Spanish and king mackerel, and 34% for red snapper, respectively. These results show that combined alternatives of bycatch reduction were effective and must be considered in order to minimize losses of shrimp catch associated with implementation of BRD, or fishing effort restrictions.


Agriculture, Fisheries and Aquaculture

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