Determinants of the nursery role of seagrass meadows in the sub-tropical Gulf of Mexico: inshore-offshore connectivity for snapper and grouper

Quantifying the nursery role of habitats or locations in supporting fisheries is central to understanding population-scale animal-habitat relationships, and in guiding ecosystem-based management. We assessed the nursery role of northern Gulf of Mexico seagrass meadows for gray snapper, lane snapper, and gag recruiting to Alabama’s extensive offshore reef complex. We accomplished this using broadscale juvenile trawl surveys and geochemical tags—indicative of past habitat use—stored in the otoliths of >2200 fishes. These natural tags revealed that 47-61% of snapper and gag recruits to Alabama reefs originated in Florida panhandle seagrass nurseries. Seagrass meadows in Alabama and Mississippi were also important nurseries for snappers and gag, contributing 26-46% of recruits. Despite high juvenile snapper and gag catches along the extensive Chandeleur Islands, Louisiana, relatively few of those fishes recruited to Alabama’s reefs (<13% of total recruits, across species), although they may have recruited to populations outside our sampling domain. Beyond the applied value of these data for resource management (i.e. interstate connectivity), our findings highlight broadscale drivers of the nursery role of juvenile habitats for coastal marine populations. These factors include: (1) juvenile habitat extent (i.e. extensive Florida panhandle meadows sourced the most recruits for Alabama fisheries); (2) proximity between juvenile and adult habitats (i.e. highest unit-area contribution from Alabama-Mississippi meadows); and (3) unidirectional, alongshore migration of egressing juveniles (i.e. primarily east-to-west movement, enhancing connectivity with Florida panhandle nurseries, and dampening connectivity with Chandeleur nurseries).

INFLUENCES OF UPPER FLORIDAN AQUIFER WATERS ON RADIOCARBON IN THE OTOLITHS OF GRAY SNAPPER (Lutjanus griseus) IN THE GULF OF MEXICO

ABSTRACTThe otoliths (ear stones) of fishes are commonly used to describe the age and growth of marine and freshwater fishes. These non-skeletal structures are fortuitous in their utility by being composed of mostly inorganic carbonate that is inert through the life of the fish. This conserved record functions like an environmental chronometer and bomb-produced radiocarbon (14C)—a 14C signal created by atmospheric testing of thermonuclear devices—can be used as a time-specific marker in validating fish age. However, complications from the hydrogeology of nearshore marine environments can complicate 14C levels, as was the case with gray snapper (Lutjanus griseus) along the Gulf of Mexico coast of Florida. Radiocarbon of these nearshore waters is influenced by freshwater input from the karst topography of the Upper Floridan Aquifer—estuarine waters that are 14C-depleted from surface and groundwater inputs. Some gray snapper likely recruited to this kind of environment where 14C levels were depleted in the earliest otolith growth, although age was validated for individuals that were not exposed to 14C-depleted waters to an age of at least 25 years with support for a 30-year lifespan.

Gene flow between subpopulations of gray snapper (Lutjanus griseus) from the Caribbean and Gulf of Mexico

Background The gray snapper (Lutjanus griseus) has a tropical and subtropical distribution. In much of its range this species represents one of the most important fishery resources because of its high quality meat and market value. Due to this, this species is vulnerable to overfishing, and population declines have been observed in parts of its range. In recent decades, it has been established that knowing the level of genetic connectivity is useful for establishing appropriate management and conservation strategies given that genetic isolation can drive towards genetic loss. Presently the level of genetic connectivity between subpopulations of L. griseus of the southern region of the Gulf of Mexico and the Caribbean Sea remains unknown. Methods In the present study we analyze genetic structure and diversity for seven subpopulations in the southern Gulf of Mexico and the Mexican Caribbean Sea. Eight microsatellite primers of phylogenetically closely related species to L. griseus were selected. Results Total heterozygosity was 0.628 and 0.647 in the southern Gulf of Mexico and the Mexican Caribbean Sea, however, results obtained from AMOVA and RST indicated a lack of genetic difference between the major basins. We also found no association between genetic difference and geographic distance, and moderately high migration rates (Nm = > 4.1) suggesting ongoing gene flow among the subpopulations. Gene flow within the southern Gulf of Mexico appears to be stronger going from east-to-west. Conclusions Migration rates tended to be higher between subpopulations within the same basin compared to those across basins indicating some regionalization. High levels of genetic diversity and genetic flow suggest that the population is quite large; apparently, the fishing pressure has not caused a bottleneck effect.