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The coastal mink on Vancouver Island, British Columbia Halter, David Francis

Abstract

The mink (Mustela vison evagor) which frequents the Pacific coastal shores of Vancouver Island, British Columbia, forages primarily in the marine intertidal zone, feeding mostly upon small crustaceans and fish. Decapod crabs of the family Cancridae are taken throughout the year, but especially in summer when they move into intertidal waters to mate and moult. Kelp crabs (Pugettia) and most fish species appear to be most vulnerable in winter, when storms create instability in their near-shore habitat. Water depth, substrate particle size, and the degree of protection from heavy wave action are among the most important factors influencing the success of mink hunting for these organisms. Along these food-rich shores, most mink hunted at success rates which would have provided their daily requirements in less than two hours of hunting activity. Nevertheless, observations of individuals which hunted with less than average success, under various conditions, indicate that accessibility to food varies with place and time, especially relative to tide level. Males regularly outnumbered females in all areas studied, and the proportion of juveniles in study populations was lower than expected, averaging less than two young per adult female in all seasons. The mating season, in this area, peaks during late May and the first half of June. Despite the fact that this is two months or more later than has been recorded for mink elsewhere, there is evidence that the delayed implantation characteristic of the species has been retained. Although both mating and parturition occur when general food availability and climate are favorable, the apparently short delay (10 to 15 days) does not appear to enhance this timing. It is speculated that the delay constitutes a period of convalesence for female mink between the rigors of the mating season and the demands of maternal functions. Results from fur farm studies on mink productivity, interpreted within the framework of observations from the wild situation, suggest that productivity varies inversely with the frequency of contact which female mink have with other mink before and during the mating season. Population densities ranged from about 1.5 to more than 3 animals per kilometer of shoreline on the Vancouver Island study areas. As has been the case with mink populations studied elsewhere, turnover was rapid with losses of 50 per cent or more between successive (4 month) seasons. There was little evidence of emigration and most losses are believed to have been due to mortality. Individual mink were known or suspected to have died from encounters with other mink, prey species, and predators, but there was no evidence that these were regular sources of mortality. A die-off which occurred in 1970 may have been related to a local build up of paralytic shellfish toxin. Animals examined were relatively free of parasites although sinus worms, which caused severe skull damage in some cases, and an unidentified mite, which apparently caused open wounds on the hindquarters of many mink in summer, both occurred frequently enough to have had population consequences. Summer inanition, characterized by deteriorating condition of numerous individuals and increased incidence of livetrap deaths during the period May-July, is believed to have been related to stresses imposed by reproductive activity and moulting, both of which occur at that time; this may have been the prime factor contributing to population turnover. Most individuals were relatively sedentary, ranging over only small areas even during the breeding season. Males had larger ranges (mean=0.72 km of shoreline) than females (0.41 km) and, due to a few longer dispersal movements (up to 9 km), juvenile males emerged as the most mobile class. There is evidence that range size was inversely related to the quality of local hunting habitat. Although much overlap occurred, individuals maintained distinct ranges (territories). Intrusions on portions of a territory occurred primarily when the resident territory holder was not present to assert himself, although avoidance rather than confrontation appeared to be the primary method by which the animals keep, separate. The results of encounters, and other evidence, indicate that mink society favors the adult male. Relative to population regulation, the emerging hypothesis is that all members of a population are in competition for some limited resource, probably suitable year-round hunting spots in the case of the littoral foraging mink of this study and, due to the dominance of adult males, females suffer increased mortality and decreased productivity when this competition is intense, i.e. when the population is high and/or when the sex ratio is high to males. A management implication of the hypothesized regulation mechanism is that harvest systems designed to select males are the most likely to enhance productivity.

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