Local population dynamics of estuarine blue crabs: abundance, recruitment and loss

• Published in: Marine ecology. Progress series (Halstenbek) 1992, Vol.87 (1/2), p.23-40
• Main Authors: Fitz, H. Carlton, Wiegert, Richard G.
• Format: Article
• Language: English
• Subjects: Brackish; Callinectes sapidus; Crabs; Emigration; Mark release recapture; Modeling; Population dynamics; Population estimates; Population parameters; Population size; Salt marshes; Young animals
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps087023

Due to the extensive motility and prolonged reproductive period of the blue crab Callinectes sapidus, cohorts may mix and be difficult to differentiate. In such a case, inferences about local population loss and turnover from changes in relative abundance may be tenuous: some form of identification other than size is necessary. We injected coded microwire tags into the musculature of blue crabs ≥ 35 mm carapace width and used the Jolly-Seber (J-S) mark-recapture model to estimate blue crab population size, loss probability, and recruitment over a 2.5 yr period within a 1 km2 salt marsh creek system. A separate J-S analysis on juveniles ≤ 80 mm, along with an analysis of the population size structure, enabled us to infer which blue crab size classes were responsible for the changes in population size. Juvenile blue crabs 35 to 80 mm were most common in spring 1985 and from autumn through spring 1986. Their relative abundance index generally decreased from spring through summer, as larger juveniles and adults > 80 mm became more common. Whereas the sex ratio was close to unity when the smallest juveniles were predominant, the ratio became heavily biased towards males when the population size structure shifted toward the larger sizes. Very few juveniles ≤ 80 mm recruited to the population in the autumn-spring of 1986–87 compared to the previous spring and autumn periods. Subsequently, the total population size was lower through the summer of 1987 compared to the prior 2 years. Density peaked in spring 1985 and 1986 (during periods of juvenile abundance) at approximately 0.2 ind. m−2), expressed as the no. of individuals at low tide throughout the entire subtidal area in the study site. The J-S analysis indicated the existence of time periods when recruitment and loss occurred concurrently. During many spring and summer sampling periods in 1985 and 1986, the probability of loss due to emigration or death was greater than that observed via net changes in the population size. Loss probability during 0.5 mo intervals in March through June 1986 varied from 0.04 ± 0.01 SE to 0.40 ± 0.04 (averaging 0.23) during a period of relatively stable population size of 11 700 to 15 800 individuals. Because of confounding recruitment, the realized rates of gross loss were higher than those discerned from net changes during such time periods. Most of these dynamics involved the juvenile portion of the population: we rarely recaptured large adult crabs, which had a brief residen