Determining cetacean - cephalopod trophic interactions : an isotope approach

Determining the biotic and abiotic influences on the distribution and abundance of marine mammals is essential for understanding the dynamics of the food chain. The predator-prey relationship can be deterministic in shaping both the community structure and function of marine ecosystems. This is especially pertinent to recovering toothed whale populations given their large size and high prey consumption rates. A greater knowledge of the trophic linkages between toothed whales and their prey will facilitate assessments of their combined impact on the ecosystem since marine food webs are a fusion of bottom-up and top-down energy and nutrient flow. This is of particular interest for regions that have recovering whale populations and varying climactic changes, such as Australia. Whales and dolphins strand in all Australian coastal areas. However, it is the southern states, of which Tasmania is a particular hotspot, that experience frequent strandings. In the previous two decades there has been in excess of 70 mass strandings. Two of the most common species to strand are long-finned pilot whales Globicephala melas edwardii and sperm whales Physeter macrocephalus. Until 2010, there had been 3974 of these two species that had stranded around Tasmania, 87% of which were long finned pilot whales and 13% were sperm whales (parks.tas.gov.au). Despite the frequent stranding of these toothed whales there is a paucity of trophic information for these species from the Tasmanian region. Similarly, comparatively little is known of the trophic dynamics of oceanic cephalopods which are considered a major prey of many toothed whales in this part of the world. This study used stable isotope analysis to quantify the diet and trophic relationship between toothed whales and cephalopods in regions surrounding Tasmania. Carbon (˜í¬•\\(^{13}\\)C) and nitrogen (˜í¬•\\(^{15}\\)N) isotopic analysis was conducted on cephalopods that were captured incidentally by commercial fisherman to provide a baseline with which to compare isotopic values of cephalopod prey from predator's stomachs. Isotopic values indicated that the cephalopod community was inclusive of 3 distinct trophic levels (6.7 ¬¨¬± 1.1 ‚ÄövÑ‚àû (Moroteuthis ingens) to 12.0 ¬¨¬± 0.5 ‚ÄövÑ‚àû (Idioteuthis cordiformis), ranging from lower trophic crustacean feeders to higher trophic fish feeders. Some cephalopod species provided evidence of resource partitioning while other species indicated a dietary shift from lower to higher trophic levels as they matured. Furthermore, cephalopods occupy a range of trophic levels and are therefore important vectors in transferring energy up the food chain, particularly to toothed whales. Intrinsic factors such as age, sex or lactation status exhibited little variation on skin ˜í¬•\\(^{13}\\)C and ˜í¬•\\(^{15}\\)N values of long-finned pilot whales from 3 stranding events off the coast of Tasmania. Nevertheless, small variations due to stranding events were evident. The ˜í¬•\\(^{13}\\)C and ˜í¬•\\(^{15}\\)N values suggested that some adult pilot whales may have a more demersal or shelf foraging habitat while most reflect a pelagic oceanic foraging habitat. Whales showed little trophic enrichment compared to beaks from their stomachs suggesting supplementation of their predominantly teuthophageous diet with other organisms. Long-finned pilot whales also had one of the lowest ˜í¬•\\(^{15}\\)N values (12.2 ¬¨¬± 0.4 ‚ÄövÑ‚àû) for pelagic marine mammals in the region. Isotopic analysis defined sperm whales as an apex predator in this region. However, based on skin ˜í¬•\\(^{13}\\)C and ˜í¬•\\(^{15}\\)N values, sperm whales showed low variation in foraging based on strandings. Sperm whales are largely teuthophageous feeding on oceanic squid from the meso- and bathy-pelagic zone. ˜í¬•\\(^{13}\\)C and ˜í¬•\\(^{15}\\)N values of squid beaks from their stomach contents confirmed that the whales had been foraging in an analagous isotopic region to that of subtropical waters around Tasmania. The isotopic signature of sperm whales was likely a result of a mixture of both low and high trophic level cephalopod prey, with the ˜í¬•\\(^{15}\\)N value of some prey (e.g. M. hamiltoni 16.8 ¬¨¬± 0.7 ‚ÄövÑ‚àû) exceeding that of the ˜í¬•\\(^{15}\\)N value of the predator (e.g. 14.7 ¬¨¬± 0.8 ‚ÄövÑ‚àû). Smaller-sized beaked whales had lower ˜í¬•\\(^{15}\\)N values (range 11.0 ‚ÄövÑ‚àû, Cuviers cavirostris calf to 13.2 ‚ÄövÑ‚àû, Tasmacetus shepherdi) and assumed lower trophic position than some other odontocetes from the region. There was evidence of niche separation between species. Furthermore, isotopic values of stomach contents of a Cuvier's beaked whale suggested it might not be predominantly teuthophageous. The beak ˜í¬•\\(^{15}\\)N values of all cephalopods from the stomach contents exceeded the ˜í¬•\\(^{15}\\)N values for the predator itself. Comparisons between different whale tissues of the same animal highlighted the importance of species-specific isotopic discrimination values to accurately evaluate foraging strategies. Toothed whales are good biological samplers for describing unknown cephalopod assemblages from meso- and bathypelagic water masses. Combined isotopic analysis of stomach contents with that of the predator highlighted whether oceanic cephalopods were likely to be a dominant prey item in their diet. Moreover, the importance of cephalopods as mid and higher order predators in the region and their role in transferring energy up the food chain was confirmed. However, evidence suggested that the toothed whales themselves were more generalist rather than specialist foragers.