Date of Award



© 2015 Laura N. Sirak

Document Type


Degree Name

Master of Science in Marine Sciences


Marine Science

First Advisor

Kathryn Ono

Second Advisor

Carrie Byron

Third Advisor

Steven Travis


Marine mammals interact with commercial fisheries via competition for resources, depredation (feeding on fish caught in gear), entanglement, and bycatch in fishing gear. In New England, gray seals (Halichoerus grypus) and harbor seals (Phoca vitulina) are often taken as bycatch in sink-gillnet fisheries and are believed to depredate fish in gillnets. As seal populations increase, interactions with fisheries are also likely to increase, affecting both seal stocks and the New England fishing industry. This study aims to understand seal bycatch in the New England sink-gillnet fisheries by identifying the spatial and temporal trends in bycatch as well as the characteristics of seals that are taken most frequently as bycatch. Depredation is also a concern in the commercial fishing industry, however, there is some controversy among fishermen and scientists concerning the identification of the species responsible for depredation (e.g. seal vs. spiny dogfish (Squalus acanthias)). Therefore, a protocol for identifying seal and spiny dogfish depredation was developed and used to identify depredation in a small-scale study of the sink-gillnet fishery targeting skate.

Data from the Northeast Fisheries Observer Program (NEFOP) from 2005 – 2013 were analyzed to assess seal bycatch in the Northeast sink-gillnet fishery. Male seals were taken significantly more frequently than females, with young of the year most commonly occurring as bycatch. Areas where seals were taken in New England shifted seasonally, generally following the annual life history of each seal species. Gray seal bycatch showed an increasing trend over the years of study, with highest bycatch occurring in the spring in areas closest to haul out sites: Muskeget and Monomoy Island, MA, USA. Harbor seal bycatch was much more variable between years, with highest bycatch occurring in the winter near major harbor seals haul out sites along the southern Maine coast and southeastern Massachusetts. This study was a crucial step to understanding the complexities of seal-fishery interactions in New England.

In order to mitigate damage from depredation, it is important to know the source of the damage. Characteristics of seal and spiny dogfish bites were identified using foam imprints from jaws and bites by captive animals in the soft tissue of fish. Measurements from bite imprints and damaged fish were used to develop a protocol for identifying damage in the field. In general, dogfish bites were clean (flesh completely removed), circular in shape, and wider than long (bite ratio (bite length/bite width) < 0.6), whereas seal bites were ragged (flesh not completely removed, but partially torn from the bite), rectangular or trapezoidal in shape, and usually longer than wide or equal in length and width (bite ratio > 0.7). This protocol was used to identify damaged catch observed on a commercial gill-net fishing vessel targeting skate in New England waters June – August 2014. In this small-scale study, dogfish bites were identified as the damage source significantly more frequently than seal bites (Multifactor ANOVA: F df=2,66 = 9.306, p = 0.0003; Tukey HSD: p < 0.0001). This inexpensive, quick, and practical protocol can be used on a larger scale to further understand depredation by seals and dogfish throughout New England.


Master's thesis

This digital object has been funded in part with Federal funds from the National Science Foundation, Division of Graduate Education, under Award No. #0841361, "The Interactions of Biology, Chemistry and Physics at the Land-Ocean Interface: A Systemic PARTnership Aimed at Connecting University and School (SPARTACUS)", to the University of New England.