An investigation of the effects of various environmental parameters on underwater foraging behaviour of the American mink, Mustela vison Schreber

Abstract

The aim of this study was to investigate the effects of changes in various environmental parameters on the underwater foraging behaviour of the American mink, Mustela vison. The study was conducted in an indoor pool. The effects of changes in the following parameters were investigated: (i) Water Depth. This was altered from 0.3 m to 1.65 m. (ii) Current Flow. Presence of either a deep or a surface current was compared to no current flowing. (iii) Prey Density. Four prey densities were used, 25%, 50%, 75% and 100%. (iv) Habitat Complexity. Hides were arranged in a regular, random or clumped pattern. The effects of habitat complexity were investigated in conjunction with prey density. Results are presented for gross changes in foraging behaviour, i.e. dive rate (number of dives per min), successful dive rate, hide visit dive rate, proportion of dives visiting a hide, proportion of successful dives and proportion of successful hide visits, and for finer changes within each dive, i.e. dive duration, time on bottom, number of hides visited per dive, mean time in hide, proportion of time on bottom spent hide searching, distance travelled underwater, proportion of direct to indirect dives, mean number of turns per dive, directionality of dives and revisiting of hides. The results showed that as depth increased, animals made fewer dives, but the dives were of longer duration. The extra time on bottom appeared to be used for locating hideswhich could no longer be located aerially before diving. Deep current was found to be not strong enough to seriously affect foraging behaviour. However, the surface disruption caused by the surface current, led to an increased dive rate, possibly in an attempt to locate hides that could no longer be located aerially, although other parameters such as proportion of dives visiting hides, dive duration etc., were generally unaffected by current flow. The conclusion was that mink were well able to continue foraging with current speeds of up to 0.86 m s(^-1). It was found that as prey density increased, animals, generally, made fewer dives of shorter duration, more of which were successful, although there was considerable individual variation. For habitat complexity, animals, generally, behaved similarly if hides were arranged randomly or in clumps, but when hides were regularly distributed, fewer hide visit dives were performed. However, mean time on bottom tended to be longer, resulting in little difference in foraging efficiency between the three conditions. A brief review of individual strategies revealed that there were considerable individual differences in foraging strategy. These were not related to sex, thus, some individuals consistently used a strategy of many short duration dives, generally visiting only one hide per dive. Others opted for fewer, longer duration, dives, generally involving more than one hide visit. Further, mean dive duration was not related to body weight. An investigation into the maximum underwater swimming speed achieved by mink showed that animals could reach speeds of over 1 m s(^-1). However, comparison with swimming speeds of fish species preyed on by mink, revealed that the fish swam faster. A review of the habits of the fish eaten, however, revealed that most were sedentary, bottom dwellers. The implications from this are that commercially important fish, e.g. salmon and trout, may well be taken mainly as diseased or spent individuals. The overall conclusion reached was that mink are highly versatile mustelids, and have 'specialised' in the ability to utilize both terrestrial and aquatic habitats.