Our results suggest that the consumption of several prey categories fluctuates significantly year to year. Few data are available to indicate abundance of the main prey categories, although fishery statistical

data from ICES subarea IX (west of the Iberian Peninsula) suggest that ommastrephid (virtually all of which will be Illex coindetii and Todaropsis eblanae, Pierce et al. 2010b) abundance has fluctuated widely. Landings in the early 1990s were low, as little as 250 tons LBH589 in 1993, before rising to a peak of almost 3,000 tons in 1997 before declining again reach slightly over 300 tons in 2007. A similar trend was seen in Bay of Biscay waters (ICES 2000, 2011). Our dietary data are clearly inadequate to test whether diet has tracked prey abundance, selleck but there was evidence of a decline in the numerical importance of Illex and Todaropsis in pilot whale diet during approximately 2000 to 2005. The higher importance of octopus in the diet of pilot whales found in the present study (and by Spitz et al. 2011) compared to most previous studies probably reflects a latitudinal trend, with squids (mainly ommastrephids) dominating the diet at higher latitudes

while octopods are more important at lower latitudes. These differences could relate to differences in prey availability, but there are no relevant abundance estimates for these cephalopod groups and this hypothesis is not presently testable. Improving our knowledge of the factors affecting the diet of deep divers such as pilot whales could help us to understand the trophic links within these systems and also the relationships between oceanic and shelf waters that this predator seems to be able to exploit simultaneously.

It would be interesting to understand why the whales appear to take mostly prey species of relatively low energy density. Few data exist on the calorific values of oceanic cephalopods although some figures are available for MCE neritic species. For example, Spitz et al. (2011) gave values of 4.7 kJ/g for E. cirrhosa and 4.4 kJ/g for squid of the family Ommastrephidae (only Illex coindetti and Todaropsis eblanae were analyzed). These values are similar to those for fish of the family Gadidae but are quite low when compared with the energetic content of some other fish such as clupeids and some myctophids. In principle, diet selection is expected to reflect a trade-off between calorific content of the prey and the energetic cost of capturing them, suggesting that prey species such as Eledone cirrhosa may be particularly abundant and/or easy to capture. However, it is also true that not all biases can be accounted for when inferring the diet of a species by the analysis of the stomach contents of stranded individuals, e.g.