Effects of sport fishing on harbor seal hunting success

Competitive interactions between marine mammals and fisheries are well documented. For example, pinnipeds (seals and sea lions) may impede the recovery of commercial fish stocks, and fisheries can negatively affect seals via bycatch and by diminishing food availability. However, the interactions between pinnipeds and sport fishers are not well documented, despite the fact that both utilize the same resources. I aimed to investigate whether sport fishermen affect the hunting success of harbor seals (Phoca vitulina). Whatcom Creek, a small and accessible river located in downtown Bellingham, WA, is a common place for sport fishers and harbor seals to aggregate and capture returning salmon (Oncorhynchus spp.). This makes it an ideal system to examine the relationship between seals and sport fishers. Students from Western Washington University have been recording seal behavior, hunting success, and counts of sport fishers and seals at the creek since 2011. With a linear model, I used this data combined with salmon arrival numbers from the Whatcom Creek hatchery to determine which factors are most associated with hunting success (number of fish caught by seals). This model included the number of seals, number of fishermen, and number of salmon arrivals as fixed factors, and year as a random factor. I only included days with over 100 salmon arrivals during September-December, when adult salmon return. The model best supported by the data included only the effect of the number of seals present. These results suggest that seals could be working together when foraging in the creek, increasing hunting success. Alternatively, the number of seals could just be a better indicator of the number of salmon available, and could have no effect on hunting success. In any case, there appears to be no effect of sport fishermen on the hunting success of harbor seals.