Options for modulating intra-specific competition in colonial pinnipeds: the case of harbour seals (Phoca vitulina) in the Wadden Sea

Abstract

Colonial pinnipeds may be subject to substantial consumptive competition because they are large, slow-moving central place foragers. We examined possible mechanisms for reducing this competition by examining the diving behaviour of harbour seals (Phoca vitulina) after equipping 34 seals (11 females, 23 males) foraging from three locations; Rømø, Denmark and Lorenzenplate and Helgoland, Germany, in the Wadden Sea area with time-depth recorders. Analysis of 319,021 dives revealed little between-colony variation but appreciable inter-sex differences, with males diving deeper than females, but for shorter periods. Males also had higher vertical descent rates. This result suggests that males may have higher overall swim speeds, which would increase higher oxygen consumption, and may explain the shorter dive durations compared to females. Intersex variation in swim speed alone is predicted to lead to fundamental differences in the time use of three-dimensional space, which may help reduce consumptive competition in harbour seals and other colonial pinnipeds.

Drivers and constraints on offshore foraging in harbour seals

“…Our understanding of harbour seal foraging behaviour has largely been drawn from inferences based on surface movement patterns and dive shape acquired at low sampling rates from biotelemetry tags 23 . Although fine-scale movement sensors (accelerometers and magnetometers) have previously been used on harbour seals in the German Wadden Sea 8 , sampling rates were too low (maximum of 1 Hz) to detect the fast movements associated with prey capture attempts 37 . Here we combine high-resolution bio-logging data from GPS, depth, accelerometer and magnetometer sensors to examine fine-scale harbour seal foraging and resting behaviour during multi-day offshore foraging trips.…”

“…Harbour seals ( Phoca vitulina ) are primarily a coastal species but also travel offshore for days at a time, often journeying > 100 kms from haul-out sites 6 – 8 . However, some individuals target resources closer to the haul-out 6 , 8 , 9 , raising the question: why do some individuals accept the extra travel costs associated with foraging further offshore?…”

“…Harbour seals ( Phoca vitulina ) are primarily a coastal species but also travel offshore for days at a time, often journeying > 100 kms from haul-out sites 6 – 8 . However, some individuals target resources closer to the haul-out 6 , 8 , 9 , raising the question: why do some individuals accept the extra travel costs associated with foraging further offshore? Indeed, it has been suggested that some populations, such as the Wadden Sea harbour seals, feed almost exclusively at offshore sites suggesting that these sites provide a resource which is not available closer to the haul-out 6 , 8 , 10 , 11 .…”

“…Accelerometers have been employed on a number of pinniped species to detect prey capture attempts 33 , 36 , 37 and sharp changes in acceleration appear to be reliable indicators of both suction and raptorial feeding in harbour seals 37 , making this a useful sensor to quantify foraging rates. Combining data from multiple sensors can provide a more definitive picture of an animal’s fine-scale movements and the context within which they are performed 8 , 38 – 40 .…”

Bio-telemetry as an Essential Tool in Movement Ecology and Marine Conservation

“…Another, more frequently observed defining intrinsic factor of foraging efficiency is sex, which can be observed in many taxa as a possible mechanism to reduce intra-specific competition. Sexually distinct foraging strategies were observed, amongst others, in lesser black-backed gulls Larus fuscus and harbor seals Phoca vitulina (Camphuijsen et al 2015;Wilson et al 2015b). A healthy body condition is also vital for efficient foraging.…”