Estimation of Red Snapper Abundance on Oil and Gas Platforms in the northern Gulf of Mexico based on Mark Recapture Methods

Abstract

Estimating abundance of sport and commercial reef fish species remains difficult due to cryptic habitat use patterns. One such species is Red Snapper, Lutjanus campechanus, which is perhaps the most important fish species in the Gulf of Mexico. It is known that Red Snapper are an important component of fish assemblages on oil and gas platforms, yet there are few quantitative abundance estimates for this species on platforms. This lack of abundance information for Red Snapper on these structures has led to questions concerning the required removal of platforms after production has ceased, i.e., if platforms are removed will the loss of reef habitat and associated Red Snapper populations cause further reductions on a stock that is already heavily fished. To address this question, the present study applied a Petersen mark-recapture study with Bailey’s modification to estimate Red Snapper abundance on platforms in the northern Gulf of Mexico. Abundance estimates were based on the number of tagged Red Snapper available for recapture, the number of recaptures, and the total number caught at the time of recapture efforts. Red Snapper were captured with hook and line (8/0 circle hooks), weighed (nearest 0.1 kg), measured (fork length, standard length, total length mm), tagged with Floy internal anchor tags, and released at 20 different platforms on the continental shelf in the northern Gulf of Mexico (Texas, Louisiana, and Alabama) from February 2017 through October 2018. To allow for full recovery of Red Snapper from tagging effects there was a minimum time period of one week between mark and recapture efforts. The number of tagged Red Snapper available for recapture on each platform was adjusted for emigration, tagging artifacts, natural mortality, fisher non-reporting, and tag retention based on a separate Red Snapper telemetry study on the same study platforms. The adjusted Bailey’s mean (±SE) abundance estimate = 1,228 ± 602 with a mean weight (±SE) = 1.63 ± 0.04 kg and ranged from 107 to 10,698 Red Snapper per platform. The adjusted abundance of Red Snapper by platform were compared among geographical region (continental shelf areas off Alabama, Louisiana, and Texas), depth zone, distance-from-shore, and year. An interaction of region and year had a significant effect on Red Snapper abundance. Depth and distance-from-shore had significant correlations with Red Snapper abundance on platforms, but these results were most likely due to differences in platform size. Mean fishing mortality (F) of Red Snapper on all sampled platforms was low compared to other platform studies with F = 0.19 (0.11 - 0.25; 95 % CL) and ranged from 0 to 0.47 among platforms. Red Snapper tagged on platforms in 2017 had an F = 0.32 (0.23 – 0.44, 95 % CL) and Red Snapper tagged on platforms in 2018 also had an F = 0.32 (0.17 – 0.55, 95 % CL). There were 533 platforms at 18 to 46 m depths in the northern Gulf of Mexico between 2017 - 2018. Thus, based on the adjusted mean Bailey’s abundance per platform there were 654,524 Red Snapper with a weight of 1.05 million kg on platforms at these depths during the present study time period. This total abundance and weight estimate indicated that Red Snapper on platforms accounted for 5% of the 2016 Gulf of Mexico Red Snapper biomass. Thus, it is unlikely that the explosive removal of platforms will have a significant effect on Red Snapper stock size in the Gulf of Mexico.