Young-Of-The-Year Stone Crab (Genus Menippe) Recruitment in the Gulf of Mexico Off Florida: Key Shallow-Water Hotspots

Abstract

Knowledge of juvenile recruitment (defined here as the quantitative addition of early benthic life stages to a local population) is important for conserving heavily harvested species and their critical habitats. Stone crabs (genus Menippe) are commercially and ecologically important throughout the Gulf of Mexico nearshore waters off Florida, but very little is known about their recruitment. Using standard commercial stone crab traps, megalopal and postsettlement juvenile (collectively, young-of-the-year, “YOY”) stone crabs were sampled at multiple spatial scales and for multiple stone crab generations to characterize geographical, seasonal, and interannual variation in their distribution and relative abundance (numbers collected on traps) in the Gulf of Mexico off peninsular Florida and north of the Florida Keys (the “Florida Gulf”). The influences of potentially relevant oceanographic and biological variables [temperature, salinity, benthic community on the traps (trap fouling community), depth, distance from shore] on YOY stone crabs were investigated at locations distributed throughout the study area. Trap fouling communities had never been analyzed in detail prior to this study; an importance index was developed to quantify the seasonality, commonality, and density of the fouling communities and organisms composing the communities. Continuous, long-term data from the Tampa Bay location allowed investigation of the effects of biological relationships [relative abundance of female Menippe carrying eggs (ovigerous), occurrences of red tide (Karenia brevis, a toxic dinoflagellate) blooms] and meteorological events [tropical cyclones, El Niño Southern Oscillation (ENSO) occurrences] on temporal patterns of variation in recruitment. High relative abundances of YOY stone crabs were collected off peninsular Florida from dense, complex benthic biota that grew on crab traps located off large, pristine estuaries in relatively turbid water less than 5 m deep, where salinity ranged 24–36 and water temperature averaged 29°C–32°C. Two major recruitment locations, stable through decades, consistently accounted for approximately 50% of the YOY stone crabs collected; two secondary recruitment locations similarly accounted for another 25%. Relative abundance of YOY stone crabs was highest nearshore at the major recruitment locations, particularly during years of high relative abundance. Approximately 65%–75% of the recruits were collected August to October; however, the timing of both peak relative abundance and lowest relative abundance shifted to later in the year as latitude decreased. Patterns of change among months in relative abundance differed among locations within years and among years within locations. Water temperature, salinity, and trap fouling community were the important determinants of temporal and spatial variation in YOY stone crab relative abundance; depth and distance from shore were also important in areas where they varied notably among stations within locations and across broader expanses of the study area. At Tampa Bay, a sharp increase in relative abundance of ovigerous females in spring was followed by a similarly sharp increase in YOY stone crab relative abundance (principally stage 3–5 crabs) 3 mo later; whereas a sharp decrease in ovigerous female relative abundance in autumn was followed by a similarly sharp decrease in YOY stone crab relative abundance 1 mo later. Coincidence of the normal autumnal decrease in YOY stone crab relative abundance with red tides and tropical cyclones prevented assigning clear relationships between seasonal change in relative abundance and these potential external influences. Annual decreases in relative abundance were significantly related to the occurrence of tropical cyclones that came near Tampa Bay during the previous year. Twice, confluences of multiple hurricanes, timely red tides, and ENSO events were followed by nearly complete, 1- to 2-y recruitment collapses. In contrast, a single meteorologically and oceanographically highly anomalous year coincided with exceptionally high abundances of both YOY and ovigerous female stone crabs. A relationship between YOY stone crabs and subsequent fishery harvest was not evident, possibly due to a variety of fishery practices. Because stone crabs are subjected to intense fishing pressure throughout the Florida Gulf, the YOY stone crab recruitment grounds and their associated estuaries should be protected from the effects of development, agriculture, aquaculture, and commercial and recreational crabbing.