"Plundering Hook-Nosed Sea Pigs": Accounting for Seal Depredation in Analyses of Catch Data

Marine mammal depredation can have significant negative effects on the performance of fixed fishing gears.  Gray seals (Halichoerus grypus, Latin: ‘hook-nosed pig-of-the-sea’) feed on commercial fish weir catches in Nantucket Sound (Massachusetts, USA), and partially consumed longfin inshore squid (Loligo pealeii) and finfish in the nets are attributable to seal depredation.  A Dual-frequency Identification Sonar (DIDSON) was deployed for six ~24 hour periods in a weir in May and June 2009 to monitor diel patterns of squid and fish catches, as well as seal presence and behavior.  Daily squid and finfish landings, discards, partially consumed catch, and fishing effort were documented using logbook data from weirs in May 2007 and 2008.  Seawater temperature was recorded using data loggers affixed to individual weirs, and meteorological data (wind speed/direction) were collected from a nearby weather station.  Partially consumed catch documented in logbooks was used as a proxy for a depredation event and incorporated as a factor in analysis of environmental effects on daily squid catch rates.  Generalized additive models were applied to examine the relative influence of environmental variables, seal depredation, and date.  Seal occurrence in the weir was observed throughout 24-hour periods, with the greatest number of events occurring at night.  Behavioral observations indicated that seal presence may negatively affect the efficiency of the weir due to disruption of the passage of schooling squid and finfish into the catch chamber.  Including depredation as a factor in GAMs did not improve model performance.  Environmental variables had a more pronounced effect (greater explained deviance) in models that did not include data collected on days during which depredation was observed.  Therefore, depredation should be considered in analysis of catch rates from fixed gear.