Determining prey availability for rock lobsters, jasus lalandii: diver-sampling versus monochrome video photography

Accurate determination of diet is important in ecological studies. Many mac robenthic predators fragment their prey so finely that identification of co mponent organisms is difficult. Knowledge of the food available to masticat ory predators such as lobsters can help in determining potential prey and e nhance the accuracy of dietary assessments. With SCUBA divers limited by de pth and submersion time, benthic analyses are becoming increasingly reliant on camera systems. Here, based on paired samples, we assess the relative m erits of using removal-sampling by divers Versus monochrome video photograp hy to determine the prey spectrum available to macrobenthic predators. We a lso relate the results to the gut contents of rock lobsters (Jasus lalandii ) that were collected simultaneously. Diver samples took on average four ti mes longer to collect and process than video-image samples. No significant differences were evident between the number of species identified in diver samples and video-image samples (n = 21 paired samples, Student's t = 0.233 , P > 0.1), although an analysis of similarity (ANOSIM) test revealed that the results obtained from the two sampling methods were significantly diffe rent in terms of species composition (global R = 0.203, P < 0.01). Cluster analysis based on Bray-Curtis similarity to compare diver and video-image s amples revealed four clusters of samples with <35% similarity, confirming t hat the two techniques do sample different assemblages of species. Video im ages detected and allowed identification of all species of prey recorded in the rock lobsters' stomach contents, probably because rock lobsters prey m ainly on relatively large prey which are readily detected by video. Diver-s ampling underestimated or failed to detect two important prey types, namely small barnacles and encrusting coralline algae. We concluded that the came ra system employed was adequate for assessing the prey assemblages availabl e to macrobenthic predators such as lobsters, and that it was considerably more economical than using divers.