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In dietary studies of marine predators that have consumed cephalopods, it is first necessary to identify the cephalopod prey species using either body or beak morphology. The second step is to determine the size of the cephalopod prey consumed and to quantify the amount eating in a certain period of time. Intact cephalopods that have been removed from stomachs with little or no damage e.g. from digestion can be measured directly using dorsal mantle length and total body weight. The size of other cephalopods which are in a highly digested state, or are represented only by beak remains, must be determined using other methods. Beak measurements (specifically, lower rostral length LRL) can be used to estimate the size and weight of cephalopod prey when the relationship between cephalopod size and beak size is known. To do this, it is first necessary to remove beaks from a number of intact specimens which have been identified using whole body morphology, and then to record the relationship between cephalopod size and beak size to produce regression equations. We have obtained regression equations for 20 species of cephalopod, provided in the table below. Some species are represented by few examined specimens, and therefore it is important to consider two points before estimating body size and weight from beak size using these equations:

1. regression equations cannot be used for beaks which are outside the range of LRL lengths provided in the table below

2. you should check number of examined beaks (n) and also the correlation coefficients (r2) . Lower r2values indicate that the reliability of estimated size and weight is lower.   

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