Blue Crab (Callinectes sapidus) Population Structure in Southern New England Tidal Rivers: Patterns of Shallow-Water, Unvegetated Habitat Use and Quality

The blue crab, Callinectes sapidus, has a broad geographic distribution encompassing coastal waters of the eastern USA and Gulf of Mexico, but intraspecific patterns of habitat use and quality are lacking at northern latitudes. This study examined the population structure of blue crabs in the Seekonk and Taunton Rivers (Rhode Island and Massachusetts, USA): two tidally influenced rivers contiguous with the Narragansett Bay Estuary and dominated by shallow-water, unvegetated habitats. Crabs were collected fortnightly from May through August (2012-2016), and abundance- and growth-based metrics were used to assess riverine habitat use and quality. These metrics were also analyzed with respect to crab life history traits and in situ abiotic conditions to elucidate patterns of habitat selection throughout ontogeny. Crabs measuring 8 to 185 mm carapace width (CW; n = 2577) were collected, and two distinct age classes occupied the rivers during the spring and summer (maximum abundance similar to 5 crabs/10 m(2)). The younger age-0+ cohort was numerically dominant (similar to 88% of total catch) and comprised of male and juvenile female crabs (mean +/- SD abundance = 0.28 +/- 0.26 males/10 m(2) and 0.14 +/- 0.12 juvenile females/10 m(2)). Males accounted for the majority of age-1+ crabs (similar to 83% of cohort), yet sexually mature females were also observed (9% of cohort; mean +/- SD abundance = 0.04 +/- 0.06 adult females/100 m(2); size at 50% maturity +/- 95 CI = 129.0 +/- 0.2 mm CW). Crabs were spatially segregated along a salinity gradient with males and juvenile females prevalent in oligohaline waters (upper river salinity similar to 5 ppt) and adult females mainly concentrating in higher salinity areas (mid- and lower-river salinity similar to 11-21 ppt). Seasonal and interannual patterns in crab abundance also differed by sex and ontogeny. Peak catches of males and juvenile females occurred during the spring and mid-summer, and annual abundances were positively related to dissolved oxygen (DO) concentrations. In contrast, mature females were most abundant during August and years with elevated water temperatures. The absolute and relative growth rates of juvenile crabs equaled 0.9 +/- 0.3 mm CW/day and 1.5 +/- 0.6% CW/day, respectively, and were directly related to DO levels. A synoptic examination of crab abundance and growth across a broad geographic range indicated that shallow-water, unvegetated habitats presently serve as functional nurseries in southern New England tidal rivers.

Automated summary

The study examined the population structure of blue crabs in Rhode Island and Massachusetts, revealing two distinct age classes occupying the rivers during spring and summer. Different age and sex groups of crabs were spatially segregated along a salinity gradient, and abundance and growth rates were related to dissolved oxygen levels and water temperature.