Intraspecific competition is an important mechanism of density dependent population regulation. In many species, juvenile age classes are the least proficient foragers and lose competitive interactions with older age classes. In these situations, they may be forced to use marginal or less accessible habitat, potentially increasing juvenile mortality. We tagged grey seals (Halichoerus grypus) at Sable Island, Nova Scotia, where a breeding colony has grown from a population of less than 1000 in 1960 to around 300000 today. We tagged 24 juveniles (5 month old) with Argos satellite transmitters to track their at-sea movements and compared their behaviour to 84 adults to test for differences in habitat use consistent with intraspecific competition. We analyzed the Argos data using a behaviour discriminating state-space model which objectively categorized locations into two behavioural states based on turn angle and autocorrelation. We nominally called these states “foraging” and “transiting,” and subsequently used behavioural state to divide foraging trips into foraging and transiting segments from which a number of metrics could be tested for differences.
Results/Conclusions
Foraging trips made by most individuals contained a clear outbound transiting segment, a foraging bout, and a return inbound transiting segment. During summer months when prey appear abundant, juveniles transited longer than adults before initiating a foraging bout and juvenile foraging bouts were farther from haulout sites than adults. In addition, juveniles did not forage within 5 km the colony, an area used heavily by adult females. These observations might be explained by niche separation, predator avoidance, or differential energy requirement. However, the pattern is most consistent with displacement of juveniles by intraspecific competition from adults. These behavioural observation are also consistent with demographic observations that suggest increased juvenile mortality and a population approaching carrying capacity. These include population growth that has recently switched from exponential to logistic growth and greatly reduced recruitment of juveniles into the breeding population.
