Not to be cited without prior reference to the authors THEME SESSION (M) Impact of Cephalopods in. the Food Chain and Their Interaction with the Environment ICES CM 19981M:8 The Importance of Cephalopods in the Diets of Marine Mammals and Other Top Predators M.R. Clarke, M.B. Santos and GJ. Pierce The importance of cephalopods in the diets of top predators is reviewed, with particular reference to studies by the authors on sperm whales (Physeter macrocephalus), common dolphins (Delphinus delphis), blue sharks (Prionace glauca) and swordfish (Xiphias gladius). The use of such data to estimate cephalopod consumption by top predator populations is illustrated, as is the potential for estimating cephalopod population biomass. Cephalopods have been shown to constitute an important part of the diet of many top predator species, although the importance of cephalopods varies with area (e.g. comparing the Antarctic and NE Atlantic). Quantifying the importance of cephalopods in diets presents some methodological problems due to, e.g., differing passage rates and digestibility of fish and cephalopods. These uncertainties and biases can significantly affect calculations of the amount of food consumed. Further problems are encountered when estimating population consumption, e.g. due to uncertainty in population size estimates. Confidence limits for consumption of cephalopods by sperm whales in the NE Atlantic are estimated using bootstrap procedures. Lower and upper 95% confidence limits, based on simple assumptions about likely random errors in parameter estimates, can differ by an order of magnitude. Computations such as those used here are useful in showing where data is most imperfect. At present, the multiplicity of potential errors in any but the most basic calculations will extend confidence limits beyond the point that their quantification is helpful. However, within distinctly prescribed geographic areas, where populations are well known, modelling of species interactions may be productive. <t KEYWORDS: cephalopods, top predators, marine mammals, diet, food consumption, fisheries MR Clarke: '~ncarva", Southdown, Millbrook, Torpoint, Cornwall, PLIO 1EZ, UK [e-mail: [email protected] ME. Santos and G.J. Pierce: Department of Zoology, University of Aberdeen, Tillydrone Avenue, AB24 2IZ, UK [tel: +44 1224 272459, fax: +44 1224272396, e-mail: [email protected]@abdn.ac.ukJ --~ Introduction Analyses of stomach contents of 'top marine predators can be useful for many different investigations. Besides indicating what a predator depends on for food, the predator's distribution, diving prowess etc., it can also tell us more about the ecology of the prey species, their distribution (Clarke, 1980), seasonal fluctuations and sometimes their growth (Clarke, 1993). Extensions of such analyses havepotential for giving information on the biomass of the prey (Clarke, 1987.), on the interaction of predators with other species such as commercial species offish (Smale, 1996.) and on fluctuations of what is present in an area. Here, we are mainly concerned with cephalopods in the diets of odontocetes although mention will be made of teuthophagous birds, seals and large fish. What can be deduced from the basic data derived from examining stomach contents very much depends on our knowledge of the cephalopods and predators concerned and whether the predator being considered also includes fish and/or crustaceans in the diet. The material we obtain from stomachs varies from complete prey animals to just eye lenses. Each type of material will help us to answer different questions and provide progress towards all or some of the aims outlined above. Each has different problems; usually there is little flesh left of many oceanic cephalopods since they are digested rapidly, more rapidly than fish muscle. Cephalopod beaks are largely undigested and .sometimes accumulate in the stomach while many otoliths and bones of fish are digested and are lost from the stomach before the beaks. Fish scales, finrays and teeth can all be used for identification. Cephalopod pens and spermatophores can aid identification but the former are usually broken up early in the digestive process and the latter have not been used. Eye lenses are digested only slowly although those of cephalopods break up much more quickly than those of fish and the ratio between them is not a good guide to the ratio in the diet (Clarke et at., 1995) although their presence sometimes shows cephalopod presence in an otherwise heavily piscivorous diet. Here we shall co.nsider some of the sources of error in estimating the importance of cephalopods in the diet of cetaceans and other top predators. Our measure of "importance" will be biomass and our data will come from our own published and unpublished work. We hope to show limitations of present information and potential for further work. Dietary analysis can only provide representative results if the sample is representative. Since many current studies rely on strandings and by-catches, this is difficult to ensure (or even to test since, for some species, the only information on popUlation structure comes from strandings ). To assess 'importance" of cephalopods, in general and for species, requires first the identification of cephalopods and their beaks .. As few cephalopods are found in a sufficiently undigested state to be identified, the most efficient utilisation of samples involves identification of the beaks (as many as 18,000 beaks can be present in a sperm whale stomach, Clarke, 1980). Identification to as Iowa taxon as possible facilitates calculation of mass since size of beaks to mass relationships differ markedly according to.genus and sometimes species (and in a few families according to. sex, e.g. Onychoteuthidae). Identifying genera is usually possible from the lower beaks, for specified areas such as.the North Atlantic and many genera often include but one species in a region. Although some beaks remain unidentified in almost any oceanic region, these are not among the most numerous species and difficulties reflect taxonomic problems due to inadequacy in sampling cephalopods from oceanic waters. 2 Directly from identification we can assess frequency of occurrence and relative numerical occurrence (%) which give a 'feel' for the relative importance of the species. Estimates of the relationship between beak length (lower rostral length and lower crest or lower hood lengths- LRL, LCL, LHL, respectively) and body mass could be greatly improved for most genera. Particularly serious errors can arise from extrapolation from small animals (which are caught in nets and therefore available) to much larger animals in the food of the larger predators such as whales, seals or large fish (which are rarely caught by fishing gear, Clarke, 1983). Confidence limits have been published for few of these relationships and errors in these calculations lead to errors in the biomass percentage calculations even if only one of the species in the diet is significantly incorrect. When such biomass estimates are used to find population consumption in a geographic region these errors may be compounded by other factors such as errors in the estimation of the predator's population. Another factor in these calculations is the food requirement, usually expressed as a percentage of the mass of the predator per day (ranging from 2.0 - 3.5% for sharks and marine mammals, Clarke, 1979; Lockyer, 1981; R. Clarke et al., 1988 ; Clarke et al., 1996.). Estimates from calorific values is another way of calculating consumption but, at present, we have sufficient data for few families and any errors compound the errors of biomass estimates for each prey species. While all the above difficulties are serious when a predator under study only eats cephalopods, even more difficulties arise when the predator also eats fish because, at present, we have no effective way of quantifying cephalopods relative to fish. The latter are identified from calcareous parts, mainly otoliths, which dissolve rapidly in digestive juices while the beaks of cephalopods resist digestion and pass from the stomach at unknown intervals. Thus, they are lost from the stomach at different, unknown, intervals of time. This may lead to a considerable over estimation of the importance of cephalopods in the diet unless some way of recognising the intake during a particular time period can be found. This may be possible when feeding is restricted to dark hours and digestion during the day is followed by evacuation of remains from the stomach before the next feeding period (Clarke et a!., 1996). Statistical treatment of the computations based upon beak identifications and rostral measurements has not been popular largely because the data can so obviously only be considered a rough guide to reality due to the unknowns involved. Statistical analysis would suggest some confidence in the actual numbers given. Confidence limits can be difficult to calculate in dietary analysis for several reasons. Typically diets are described in terms of the overall proportion by weight of different prey species in a set of samples. The estimated importance of a species is thus a single number derived from the whole set of samples and does not have any confidence limits (unless we have several sets of samples and can calculate a mean and variance for sets). Furthermore, weights of prey are not measured directly, but estimated from measurements of hard parts (fish otoliths, cephalopod beaks) using previously established