Dave Thompson

How long should a dive last? A simple model of foraging decisions by breath-hold divers in a patchy environment

Abstract Although diving birds and mammals can withstand extended periods under water, field studies show that most perform mainly short, aerobic dives. Theoretical studies of diving have implicitly assumed that prey acquisition increases linearly with time spent searching and have examined strategies that maximize time spent foraging. We present a simple model of diving in seals, where dive durations are influenced by the seals assessment of patch quality, but are ultimately constrained by oxygen balance. Prey encounters within a dive are assumed to be Poisson distributed and the scale of the patches is such that a predator will encounter a constant prey density during a dive. We investigated the effects of a simple giving-up rule, using recent prey encounter rate to assess patch quality. The model predicts that, for shallow dives, there should always be a net benefit from terminating dives early if no prey are encountered early in the dive. The magnitude of the benefit was highest at low patch densities. The relative gain depended on the magnitude of the travel time and the time taken to assess patch quality and the effect was reduced in deeper dives. As the time taken to decide decreased, the relative benefit increased, up to a maximum, but fell as decision time was reduced further. Frequency distributions of different aspects of dive durations from three deep-diving and two shallow-diving seal species are presented and compared with the models predictions.

Habitat use and diving behaviour of harbour seals in a coastal archipelago in Norway

Abstract Background: the harbour seal is coastal non-migratory and suitable for behavioural studies using short-range telemetry and tracking. Methods: a combination of VHF radio telemetry and underwater ultrasonic telemetry was used to obain behavioural and physiological data from 13 harbour seals tagged at an archipelago in Norway. VHF signals were used to locate seals, and they were tracked at close proximity by inflatable boats. Results: transit and foraging activity were identified based on differences in dive profiles. When foraging, all tagged seals operated solitarily, and they returned repeatedly to the same or approximately the same foraging sites. The radio tagged seals used different types of foraging habitats, ranging from shallow kelp areas located 20 km offshore to 150–200 m deep basins with muddy sea bed located a few kilometres from the respective haul-out sites. The seals were typically foraging at or close to the sea bed. Display behaviour including underwater vocalization was recorded for sexually mature males in june and July. Conclusion: the combination of VHF and ultrasonic telemetry is useful for studies of resident seals. Tracking free ranging seals at close proximity made it possible to identify and describe their resting, foraging and display areas.

How long does a dive last? Foraging decisions by breath-hold divers in a patchy environment: a test of a simple model

*Sea Mammal Research Unit, University of St AndrewsyCentre National de la Recherche Scientifique, Institut Pluridisciplinaire Hubert Curien, Departement Ecologie,Physiologie et Ethologie (DEPE)(Received 1 March 2006; initial acceptance 25 April 2006;final acceptance 26 June 2006; published online 10 July 2007; MS. number: 8866R)Many theoretical models have been proposed to explain and predict the behaviour of air-breathing diversexploiting a food resource underwater. Many field observations of the behaviour of divers do not fit withthe prediction that to maximize energetic gain divers should dive close to their aerobic diving limits. In anattempt to explain this paradox, Thompson & Fedak (2001, Animal Behaviour, 61, 286e297) proposeda model of diving behaviour that takes into account patchily distributed prey patches of varying quality.We tested this model experimentally in a simulated foraging set-up. We measured the diving behaviour ofgrey seals, Halichoerus grypus, diving to patches of varying prey density and distance from the surface. Ourresults were equivocal with respect to the model predictions. Seals responded to prey density, leaving low-quality patches earlier. However, this pattern was still evident at long dive distances, contrary to theprediction that during deep dives seals should stay at a patch regardless of prey density. While seals max-imized dive durations at high prey densities and long distances, they did not do so at short distances. Theapparent quitting strategy of the seals always produced higher net rates of energy gain than would havebeen achieved if they had remained at the foraging site up to their aerobic dive limit on every dive. Theseresults indicate that seals’ diving behaviour, particularly bottom duration, may indicate the relative preyavailability in their environment.

Eat now, pay later? Evidence of deferred food-processing costs in diving seals

Seals may delay costly physiological processes (e.g. digestion) that are incompatible with the physiological adjustments to diving until after periods of active foraging. We present unusual profiles of metabolic rate (MR) in grey seals measured during long-term simulation of foraging trips (4–5 days) that provide evidence for this. We measured extremely high MRs (up to almost seven times the baseline levels) and high heart rates during extended surface intervals, where the seals were motionless at the surface. These occurred most often during the night and occurred frequently many hours after the end of feeding bouts. The duration and amount of oxygen consumed above baseline levels during these events was correlated with the amount of food eaten, confirming that these metabolic peaks were related to the processing of food eaten during foraging periods earlier in the day. We suggest that these periods of high MR represent a payback of costs deferred during foraging.

Estimating metabolic heat loss in birds and mammals by combining infrared thermography with biophysical modelling.

Infrared thermography (IRT) is a technique that determines surface temperature based on physical laws of radiative transfer. Thermal imaging cameras have been used since the 1960s to determine the surface temperature patterns of a wide range of birds and mammals and how species regulate their surface temperature in response to different environmental conditions. As a large proportion of metabolic energy is transferred from the body to the environment as heat, biophysical models have been formulated to determine metabolic heat loss. These models are based on heat transfer equations for radiation, convection, conduction and evaporation and therefore surface temperature recorded by IRT can be used to calculate heat loss from different body regions. This approach has successfully demonstrated that in birds and mammals heat loss is regulated from poorly insulated regions of the body which are seen to be thermal windows for the dissipation of body heat. Rather than absolute measurement of metabolic heat loss, IRT and biophysical models have been most useful in estimating the relative heat loss from different body regions. Further calibration studies will improve the accuracy of models but the strength of this approach is that it is a non-invasive method of measuring the relative energy cost of an animal in response to different environments, behaviours and physiological states. It is likely that the increasing availability and portability of thermal imaging systems will lead to many new insights into the thermal physiology of endotherms.

Sink fast and swim harder! Round-trip cost-of-transport for buoyant divers.

SUMMARY Efficient locomotion between prey resources at depth and oxygen at the surface is crucial for breath-hold divers to maximize time spent in the foraging layer, and thereby net energy intake rates. The body density of divers, which changes with body condition, determines the apparent weight (buoyancy) of divers, which may affect round-trip cost-of-transport (COT) between the surface and depth. We evaluated alternative predictions from external-work and actuator-disc theory of how non-neutral buoyancy affects round-trip COT to depth, and the minimum COT speed for steady-state vertical transit. Not surprisingly, the models predict that one-way COT decreases (increases) when buoyancy aids (hinders) one-way transit. At extreme deviations from neutral buoyancy, gliding at terminal velocity is the minimum COT strategy in the direction aided by buoyancy. In the transit direction hindered by buoyancy, the external-work model predicted that minimum COT speeds would not change at greater deviations from neutral buoyancy, but minimum COT speeds were predicted to increase under the actuator disc model. As previously documented for grey seals, we found that vertical transit rates of 36 elephant seals increased in both directions as body density deviated from neutral buoyancy, indicating that actuator disc theory may more closely predict the power requirements of divers affected by gravity than an external work model. For both models, minor deviations from neutral buoyancy did not affect minimum COT speed or round-trip COT itself. However, at body-density extremes, both models predict that savings in the aided direction do not fully offset the increased COT imposed by the greater thrusting required in the hindered direction.

How fast does a seal swim? Variations in swimming behaviour under differing foraging conditions

SUMMARY The duration of breath-hold dives and the available time for foraging in submerged prey patches is ultimately constrained by oxygen balance. There is a close relationship between swim speed and oxygen utilisation, so it is likely that breath-holding divers optimise their speeds to and from the feeding patch to maximise time spent feeding at depth. Optimal foraging models suggest that transit swim speed should decrease to minimum cost of transport (MCT) speed in deeper and longer duration dives. Observations also suggest that descent and ascent swimming mode and speed may vary in response to changes in buoyancy. We measured the swimming behaviour during simulated foraging of seven captive female grey seals (two adults and five pups). Seals had to swim horizontally underwater from a breathing box to a submerged automatic feeder. The distance to the feeder and the rate of prey food delivery could be varied to simulate different feeding conditions. Diving durations and distances travelled in dives recorded during these experiments were similar to those recorded in the wild. Mean swim speed decreased significantly with increasing distance to the patch, indicating that seals adjusted their speed in response to travel distance, consistent with optimality model predictions. There was, however, no significant relationship between the transit swim speeds and prey density at the patch. Interestingly, all seals swam 10–20% faster on their way to the prey patch compared to the return to the breathing box, despite the fact that any effect of buoyancy on swimming speed should be the same in both directions. These results suggest that the swimming behaviour exhibited by foraging grey seals might be a combination of having to overcome the forces of buoyancy during vertical swimming and also of behavioural choices made by the seals.

The status of grey seals in Britain

Grey seal pup production in Scotland was estimated through annual aerial surveys of the main grey seal breeding colonies. Between 3 and 7 counts of pups were obtained for each colony at intervals through the course of the breeding season. Pup production for individual colonies was estimated from the series of counts using a maximum likelihood model. At 3 colonies, 2 in England, annual pup production was estimated using ground counts. Between the early 1960s and the early 1990s, grey seal pup production progressively increased. At colonies in the Inner and Outer Hebrides, production appeared to stabilize during the 1990s and has remained so. Pup production at colonies in Orkney and in the North Sea has continued to increase but in recent years the rate of increase has declined. This may imply that the UK grey seal population is reaching some limit to its size. The observed changes in pup production imply that some density dependent factors are affecting the British grey seal population. Changes in either juvenile survival and/or female fecundity are the most likely options. Without knowing which of these, or what combination of these factors, is operating, estimating total population size is complicated.

How Many Seals Were There? The Global Shelf Loss during the Last Glacial Maximum and Its Effect on the Size and Distribution of Grey Seal Populations

Predicting how marine mammal populations respond to habitat changes will be essential for developing conservation management strategies in the 21st century. Responses to previous environmental change may be informative in the development of predictive models. Here we describe the likely effects of the last ice age on grey seal population size and distribution. We use satellite telemetry data to define grey seal foraging habitat in terms of the temperature and depth ranges exploited by the contemporary populations. We estimate the available extent of such habitat in the North Atlantic at present (between 1.42·106 km2 and 2.07·106 km2) and at the last glacial maximum (between 4.74·104 km2 and 2.11·105 km2); taking account of glacial and seasonal sea-ice coverage, estimated reductions of sea-level (123 m) and sea surface temperature hind-casts. Most of the extensive continental shelf waters (North Sea, Baltic Sea and Scotian Shelf), currently supporting >95% of grey seals, were unavailable during the last glacial maximum. A combination of lower sea-level and extensive ice-sheets, massively increased seasonal sea-ice coverage and southerly extent of cold water would have pushed grey seals into areas with no significant shelf waters. The habitat during the last glacial maximum might have been as small as 3% of todays extent and grey seal populations may have fallen to similarly low numbers. An alternative scenario involving a major change to a pelagic or bathy-pelagic foraging niche cannot be discounted. However, hooded seals currently dominate that niche and may have excluded grey seals from such habitat. If as seems likely, the grey seal population fell to very low levels it would have remained low for several thousand years before expanding into current habitats over the past 12,000 years or so.

The status of harbour seals ( Phoca vitulina ) in the United Kingdom

The UK holds approximately 40% of the European harbour seal population, with the majority found around the coasts of Scotland. Harbour seal populations in the UK have been monitored through a series of repeated aerial surveys of animals hauled out during the annual moult in early August. This moult count is used as a consistent index of population size. Survey methods and frequencies vary. The Scottish and English east coast populations mainly haul out in tidal estuaries and are surveyed annually, using fixed wing aircraft and digital photography. Populations in north and west Scotland often haul out on rocky shores and are surveyed less frequently, using helicopters fitted with thermal imagers. Overall, the most recent minimum estimate of the UK harbour seal population is 24,250 seals of which 19,800 are in Scotland, 3,200 in England and 1,250 in Northern Ireland. The results show that the number of harbour seals in eastern England was increasing before the 1988 and 2002 phocine distemper (PDV) epizootic but has not increased since the end of the 2002 epizootic. There is also evidence of a general decline in most of the large harbour seal colonies around Scotland. The populations along the north and northwest mainland coast were an exception, with numbers appearing to be stable. Between 2001 and 2008, the population in Orkney declined by 67% and Shetland declined by 40%, indicating harbour seals in these areas experienced substantially increased mortality or very low recruitment over this period. The widespread declines, ranging from Shetland to The Wash, suggest that the causes may have been present over a large part of the North Sea and waters off western Scotland.