The Geomorphological Complexity of a New England Estuary and Its Role in Shaping Seasonal Habitat Use and Site Fidelity of Striped Bass on a Foraging Migration

The Atlantic coast striped bass (Morone saxatilis) is an abundant migratory top predator in New England coastal ecosystems. While the general northward migration of striped bass in the spring has been well documented, much less is known about finer-scale seasonal movements, estuarine habitat use, and variation in trajectories across individuals. We investigated if discontinuities in the estuarine environment (e.g., channel confluences, drop-offs, and rugosity) altered the distribution of individual striped bass within Plum Island Estuary (PIE), Massachusetts, during the summer and fall. For this, we quantified the time fish spent at select locations within PIE, how this changed across seasons, the physical characteristics of locations where fish were present or absent, and the relationship between striped bass distribution and physical conditions. Fifty striped bass (40-60 cm TL) were tagged with acoustic transmitters in June 2009. A monthly (July-October) survey of 40 listening locations throughout the estuary was conducted to detect tagged individuals. In addition, seven stationary receivers continuously recorded fish detections in select locations in PIE from June through November, in 2009 and 2010. Ranges of detection were determined at all locations.  To quantify environmental heterogeneity within PIE, (a) confluences within PIE were identified and categorized according to tributary size, (b) depth was measured along four 500-m transects at each listening location to detect drop-offs, and (c) substrate unevenness was assessed as rugosity. An individual fish stayed in PIE for an average of 50 days (range 2 -113 days). Thirty-five fish (70%) were detected in PIE for more than 30 days with substantial individual variation in distribution across estuary region and habitat type. The monthly tracking receiver survey indicated a seasonal shift in use of PIE from the upper estuary in July and August, to the middle and lower estuary in September, with a concentration of the remaining fish in the middle estuary in October. Related to physical conditions within PIE, considerable variation existed in confluence size (mean 1,800 ±2,800m, range 111 to 16,000m, n=38) and depth change at listening locations (mean 3.5m, range 1.1 to 6.8m, n=40).  The physical diversity provided multiple options for within-estuary foraging and resting sites that could be used by migratory striped bass during summer.  This evidence of seasonal fidelity to a specific estuary, heterogeneity in habitat use, and individual variation in distribution and behavior has the potential to explain striped bass growth patterns and localized, seasonal top-down impacts of mobile predators.