Developing an index of abundance for gag grouper in North Carolina

Abstract

Gag grouper (Mycteroperca microlepis) are an economically important fish in the U.S. south Atlantic. Juvenile gag grouper utilize seagrass habitat soon after larval ingress into estuarine environments. The recent stock assessment for gag grouper indicated the need for a fishery-independent index of abundance. The goal of this study was to assess the potential for a post-larval and juvenile abundance index in North Carolina. Data on post-larval gag collected during the NOAA Beaufort Inlet Bridgenet Program (weekly samples from November – May of 1986 to 2008) were examined; additionally, ichthyoplankton were sampled nightly in spring of 2007 and 2008. A 5-m otter trawl was used to sample juvenile gag in seagrass beds at 15 to 20 randomly selected stations every two weeks from June through September in both 2007 and 2008; data on seagrass species and blade densities were determined before each trawl. Age at capture, pelagic larval duration (PLD, determined from transition mark), and fertilization dates were estimated from post-larval and juvenile otolith microstructure. A single cohort was produced each year; estimated fertilization dates ranged from February through April around full and new moons supporting planned January through April fishing closures for adult gag grouper. From 1986 to 2008, weekly concentrations of post-larval gag grouper were highest from late April to mid-May with peak ingress around new moons. Juvenile gag were caught from June through September with highest catch-per-unit-effort (CPUE) in July and August. Time of year, percent seagrass coverage, seagrass species, and sound influenced CPUE of juvenile gag grouper. Growth rates of juveniles were rapid (~1.6 mm/d) during summer months and did not differ between years. The mean PLD was ~ 43 d and did not differ among collection months suggesting no effect of PLD on survival. An annual index of post-larval abundance (adjusted for lunar effects) was developed. The spawning stock biomass (SSB) from the most recent gag grouper assessment was positively correlated with this index; thus, the post-larval index could be used as a fishery-independent index of SSB.