We estimated the incidental mortality of harbour porpoise (Phocoena phocoena) in selective salmon fisheries in southern British Columbia from reports received from federal fisheries observers (2001) and license holders (1997 – 2001).
Data sheets were developed to record prevailing conditions at the time of a harbour porpoise entanglement event. These were incorporated into the existing Federal Fisheries Observer Program. Data collection occurred throughout the 2001 – commercial salmon fishing season in the coastal waters of southern British Columbia.
Neither the seine net, nor the troll salmon fisheries reported entanglements in 2001. All observer reported entanglements of small cetaceans were phocoenids and all were caught in gill nets. In total, four porpoises were incidentally caught – each in a different statistical licensing area (licensing areas 12, 21, 25, 121). Two of these four (50%), were released alive. The two by-caught harbour porpoises that could not be released alive, died in the gill nets before either the observer or the vessel’s crew were aware of the collision. When mitigative actions were effected, live release resulted. At a minimum, the porpoises sustained superficial lacerations from the rescue efforts and the gill net collision. Morphometric and scan zone data indicated that the by-caught harbour porpoise were likely solitary juveniles.
A total of 979 gill and seine net license holders were sent a questionnaire regarding their knowledge and experiences with harbour porpoise. Of these, 27.6% returned their completed questionnaires: 250 were used in the final analysis. Respondents had an average of 33.5 years fishing experience, with two-thirds of them reporting a multi-gear career. Respondents expressed a keen willingness to participate and provided details of their harbour porpoise experiences, as well as information about other small cetacean encounters.
Respondents reported a total of 14 incidents, involving 19 harbour porpoise incidentally caught between 1997 and 2001. All were with gill nets and most involved single animals. The license holders reported a 52.6% overall release rate, with a 100% release rate for those found alive. Damage to gear was usually caused by the rescue efforts, rather than by the entangled porpoises.
Human intervention was required, as none of the phocoenids were able to disentangle themselves. Porpoises appeared to respond well to human intervention and handling, as indicated by the few reports of mortality occurring during a rescue (ancillary reports prior to the 1997 – 2001 focus period). The license holder’s responses indicated that most of the incidental catch events occurred in Salmon Statistical Licensing Areas 4, 8, 12, 21 and 22.
Based on the number of by-caught porpoises reported through the observer program, the total theoretical mortality for southern BC (Statistical Areas D and E) is 20 animals per 810 boat days fished, or 80 porpoises for the 2001 fishing effort. Given that harbour porpoise accounted for all observer reported phocoenid mortality, this translates to an estimated mortality of 80 harbour porpoise for the 2001 effort. Poisson distribution 95% confidence levels determined annual phocoenid mortality between 11 and 102 animals for southern BC (salmon licensing areas D and E). It is likely that the proportions of harbour to Dall’s porpoise killed annually are variable based on differential overlap of gill net fisheries with phocoenid spatial and temporal distribution.
This estimate was based on a very small sample size and assumed that catch rates were the same across all statistical areas. However, this assumption is questionable given that harbour porpoise and fishing effort have heterogeneous distributions.
A second estimate derived from the province-wide career experiences of license holders (1997 – 2001) yielded an estimated incidental catch of 14.1 harbour porpoise per year, with an annual mortality of 6.6 animals and Poisson distribution 95% confidence levels of 23 – 61 porpoises killed annually.
Our estimates suggest that fewer than 100 harbour porpoise are killed each year by commercial salmon gill net fisheries. However, the biological significance of our estimates of mortality is unknown due to a lack of information about numbers and rates of birth, and natural mortality of BC harbour porpoise. What we do know is that fishery caused mortality of harbour porpoise continues throughout the province, including trans-boundary areas with Washington State, even in times of reduced fishing effort. This has special significance for any populations that are at risk of gear entanglement in both US and Canadian waters, and to any populations, which are small or have restricted ranges.
The fact that mortality occurs in a highly regulated fishery raises the possibility that considerably higher mortality may have occurred in times of more permissive fisheries. If so, reduced fishery related mortality today, may mean the recovery of harbour porpoise populations historically diminished by fisheries. This in turn, underlines the need for further monitoring in southern BC and implementation of standardized reporting of harbour porpoise by-catch from central and northern regions.
Approximately half of the observer and license holder reported incidentally caught porpoises were released alive. This speaks well to the efforts of gill net fishermen in British Columbia. Further efforts to reduce harbour porpoise by-catch and increase live release rates must be practical to a commercial fishing situation and must consider the effects to the efficiency of the target species fisheries.
A number of recommendations stem from our study to reduce incidental catch and increase live release rates. These include further investigation into selective gill net fishery modifications, improving observer training specific to marine mammals, rescue and release protocols and augmenting harbour porpoise biological research.
Time or area restrictions and the introduction of acoustic net alarms do not appear to be appropriate management tools at this time, due to the uncertainty associated with the estimates of mortality, the lack of knowledge about harbour porpoise biology and the apparent rarity of occurrence per boat day fished or per respondent license holder.