Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


Detectability, the probability that a species is encountered if it inhabits a site, is often overlooked in fisheries research despite its potential to obscure habitat use inferences. Detectability can be estimated using models that also provide an estimate of occupancy (Ψ), the probability that a species inhabits a site. I used these models to estimate both probabilities, and to examine factors affecting detectability and occupancy for three fishes in Chesapeake Bay tributaries: young-of-the-year striped bass (Morone saxatilis), yearling Atlantic croaker (Micropogonias undulatus), and spottail shiner (Notropis hudsonius). Occupancy models were fitted to data from a seine survey conducted during summer, 2008 and 2009, in two Chesapeake Bay tributaries. Key assumptions of occupancy models relate to the extent and timing of fish movement: sites are independent, and no site-specific emigration or immigration occurs. A mark-recapture study of striped bass, and previously published studies of Atlantic croaker and spottail shiner, suggested that these assumptions were reasonable. Detectability differed among species and variation was explained by both gear-related and environmental factors. Effective net length (i.e., the distance from shore the seine was deployed) explained variation in detectability for all species; generally, when the effective seine length exceeded 12 m, detectability was higher and less variable. Detectability varied from early to late summer for Atlantic croaker and spottail shiner but not for striped bass. This variation may be attributed to increased net avoidance by Atlantic croaker during late summer and increased relative abundance of spottail shiner due to recruitment of individuals to the gear. Occupancy of striped bass and Atlantic croaker, both of which are transient species, was high (Ψ>0.80), whereas the resident spottail shiner occupied fewer sites (Ψ=0.59±0.21; mean±SE) and occupancy varied by river (ΨMattaponi=0.36±0.11; ΨPamunkey=0.82±0.10). Occupancy models are useful to identify factors affecting detectability of fishes captured by seines in Chesapeake Bay tributaries, but other fisheries studies would benefit from sampling design modifications that maximize detectability and improve habitat-use inferences.



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