Carol E. Sparling

Metabolic rates of captive grey seals during voluntary diving

SUMMARY The energetic cost of diving in marine mammals is a difficult value to derive given the problems of assessing metabolic rate for an animal at sea. Nevertheless, it is fundamental to our understanding of the foraging strategies of air-breathers exploiting underwater food sources. We measured the metabolic rates of eight captive grey seals, voluntarily diving in a quasi-natural setting. Oxygen consumption during post-dive surface periods was measured using open-flow respirometry, and dive behaviour of the seals was recorded using time depth recorders (TDRs). Mean diving metabolic rate (DMR) for both adults and juveniles was 1.7 times the predicted standard metabolic rate of terrestrial animals of equal size. For all animals, DMR was lower than the rate of metabolism measured whilst they were resting at the waters surface. On a dive-by-dive basis, DMR decreased with dive duration but increased with mean swim speed. Regressing the maximum 5% of DMRs against dive duration resulted in a significant negative relationship that was not significantly different from the relationship between the calculated maximum rate of aerobic metabolism and dive duration, suggesting that these seals were diving within, and up to, their aerobic limits. We developed a model that allows the prediction of DMR from information on dive behaviour of the type routinely collected in telemetry studies of wild seals. The model accurately predicts DMR using behavioural data from periods of diving with known metabolism data. This model can be used to predict the at-sea metabolic rate of wild grey seals, an important input into ecosystem models.

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.

Seasonal variation in the metabolic rate and body composition of female grey seals: fat conservation prior to high-cost reproduction in a capital breeder?

Many animals rely on stored energy through periods of high energy demand or low energy availability or both. A variety of mechanisms may be employed to attain and conserve energy for such periods. Wild grey seals demonstrate seasonal patterns of energy storage and foraging behaviour that appear to maximize the allocation of energy to reproduction—a period characterized by both high energy demand and low food availability. We examined seasonal patterns in resting rates of oxygen consumption as a proxy for metabolic rate (RMR) and body composition in female grey seals (four adults and six juveniles), testing the hypothesis that adults would show seasonal changes in RMR related to the reproductive cycle but that juveniles would not. There was significant seasonal variation in rates of resting oxygen consumption of adult females, with rates being highest in the spring and declining through the summer months into autumn. This variation was not related to changes in water temperature. Adults increased in total body mass and in fat content during the same spring to autumn period that RMR declined. RMR of juveniles showed no clear seasonal patterns, but did increase with increasing mass. These data support the hypothesis that seasonal variation in RMR in female grey seals is related to the high costs of breeding.

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.

Resolving issues with environmental impact assessment of marine renewable energy installations

Growing concerns about climate change and energy security have fuelled a rapid increase in the development of offshore and marine renewable energy installations (OMREIs). The potential ecological consequences of increased use of these devices emphasises the need for high quality environmental impact assessment (EIA). We demonstrate that these processes are hampered severely, primarily because legislation does not ensure that the significance of impacts and cumulative effects are properly assessed. We highlight why the regulatory framework leads to conceptual ambiguities and propose changes which, for the most part, do not require major adjustments to standard practice. We emphasise the importance of determining the degree of confidence in impacts to permit the likelihood as well as magnitude of impacts to be quantified and propose ways in which assessment of population-level impacts could be incorporated into the EIA process. Overall, however, we argue that, instead of trying to ascertain which particular developments are responsible for tipping an already heavily degraded marine environment into an undesirable state, emphasis should be placed on better strategic assessment.

Tracking Technologies for Quantifying Marine Mammal Interactions with Tidal Turbines: Pitfalls and Possibilities

Currently, there is great uncertainty surrounding the environmental impacts of tidal turbines on marine mammals; one major concern derives from the potential for physical injury through direct contact with the moving structures of turbines. Collecting data to quantify these risks is challenging and methods for measuring movements underwater and interactions with turbines are limited. However, potential tools include a small number of cutting-edge technologies that are being used increasingly for research and monitoring; these include animal-borne telemetry, and active and passive acoustic tracking. Recent developments in these technologies are described along with their means of application in measuring fine-scale movements and avoidance or evasion responses by marine mammals around turbines. From a risk-characterization perspective, each technique can provide information to inform risk assessments or help parametrize collision risk models; however, each has its associated benefits and drawbacks and it is clear that, in isolation, none of them can provide all the data needed to address the problem. The three approaches appear highly complementary, with the strengths of one complementing the weaknesses in others; the solution to characterizing the risks posed by tidal turbines is likely to be a combination of such techniques.

Empirical measures of harbor seal behavior and avoidance of an operational tidal turbine

There is global interest in marine renewable energy from underwater tidal turbines. Due to overlap in animal habitat with locations for tidal turbines, the potential for collisions has led to concern around strike risk. Using data from tagged harbor seals collected before construction and after operation of the SeaGen tidal turbine in Northern Ireland, this study quantifies risks of an operational turbine to harbor seals by taking into account turbine characteristics, tidal state, and seal behavior. We found 68% spatial avoidance (95% C.I., 37%, 83%) by harbor seals within 200 m of the turbine. When additionally accounting for variation in seal occupancy over depth and tidal flows, there is an overall reduction in collision risk from 1.29 to 0.125 seals per tidal cycle (90.3% reduction; (95% C.I., 83%, 98%)) compared to risk calculated under assumptions of uniform habitat use. This demonstrates the need to incorporate environmental conditions to properly assess strike risk.