California sea lions interfere with striped marlin hunting behaviour in multi-species predator aggregations

Hansen MJ, Kurvers RHJM, Licht M, et al. California sea lions interfere with striped marlin hunting behaviour in multi-species predator aggregations. Philosophical Transactions of the Royal Society of London, Series B : Biological Sciences. 2023;378(1878): 20220103.The open ocean offers a suite of ecological conditions promoting the occurrence of multi-species predator aggregations. These mixed predator aggregations typically hunt large groups of relatively small and highly cohesive prey. However, the mechanisms and functions of these mixed predator aggregations are largely unknown. Even basic knowledge of whether the predator species' interactions are mutualistic, commensal or parasitic is typically missing. Moreover, recordings of attack and capture rates of marine multi-species predator aggregations, which are critical in understanding how and why these interactions have evolved, are almost completely non-existent owing to logistical challenges. Using underwater video, we quantified the attack and capture rates of two high-trophic level marine predators, California sea lions (Zalophus californianus) and striped marlin (Kajikia audax) attacking schools of fishes in the Southern California Current System, offshore the Baja California Peninsula. Recording over 5000 individual attacks across 13 fish schools, which varied in species, size and predator composition, we found that sea lions kleptoparasitized striped marlin hunts and reduced the frequency of marlin attacks and captures via interference competition. We discuss our results in the context of the phenotypic differences between the predator species and implications for a better understanding of multi-species predator aggregations. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'

Lacunae rostralis A new structure on the rostrum of sailfish Istiophorus platypterus

Recent comparative studies of billfishes Istiophoridae and Xiphiidae have provided evidence of differences in the form and function of the rostra bill among species. Here, we report the discovery of a new structure, lacuna rostralis, on the rostra of sailfish Istiophorus platypterus, which is absent on the rostra of swordfish Xiphias gladius, striped marlin Kajikia audax and blue marlin Makaira nigricans. The lacunae rostralis are small cavities that contain teeth. They were found on the ventral rostrum surface of all I. platypterus specimens examined and dorsally in half of them. Ventrally, the lacunae rostralis were most prominent in the mid section of the rostrum. Dorsally, they occurred closer to the tip. The density of lacunae rostralis increased towards the rostrum tip but, because they are smaller in size, the percentage of rostrum coverage decreased. The teeth located within the lacunae rostralis were found to be different in size, location and orientation from the previously identified micro teeth of billfish. We propose two potential functions of the lacunae rostralis that both relate to the use of the bill in feeding mechanoreception of prey before tapping it with the bill and more efficient prey handling via the creation of suction, or physical gri

The rostral micro‐tooth morphology of blue marlin, Makaira nigricans

Billfish rostra potentially have several functions, however, their role in feeding is unequivocal in some species. Recent work linked morphological variation in rostral micro‐teeth to differences in feeding behaviour in two billfish species, the striped marlin (Kajikia audax) and the sailfish (Istiophorus platypterus). Here, we present the rostral micro‐tooth morphology for a third billfish species, the blue marlin (Makaira nigricans), for which the use of the rostrum in feeding behaviour is still undocumented from systematic observations in the wild. We measured the micro‐teeth on rostrum‐tips of blue marlin, striped marlin and sailfish using a micro‐CT approach and compared the tooth morphology among the three species. This was done after an analysis of video recorded hunting behaviour of striped marlin and sailfish revealed that both species strike prey predominantly with the first third of the rostrum, which provided the justification to focus our analysis on the rostrum tips. In blue marlin, intact micro‐teeth were longer compared to striped marlin but not to sailfish. Blue marlin had a higher fraction of broken teeth than both striped marlin and sailfish, and broken teeth were distributed more evenly on the rostrum. Micro‐tooth regrowth was equally low in both marlin species, but higher in sailfish. Based on the differences and similarities in the micro‐tooth morphology between the billfish species, we discuss potential feeding related rostrum use in blue marlin. We put forward the hypothesis that blue marlin might use their rostra in high‐speed dashes as observed in striped marlin, rather than in the high‐precision rostral strikes described for sailfish, possibly focusing on larger prey organisms

Policy-centred sampling in interest group research: Lessons from the INTEREURO project

Proper sampling is the foundation for all scientific enquiry aimed at making generalizable claims about a wider set of cases. Indeed, inferential statistical analysis presupposes representative samples and units of analysis that can be considered as independent observations. Establishing a sample of issues on which lobbying may take place, which is at the same time representative of an overall population of issues and of the varying levels of conflict and political mobilization, however, is a major challenge for interest group research. Drawing on existing research practices, we discuss a series of different approaches that may be used to establish a sample of policy issues. The focus then is on the policy-centred stratified sampling procedure used in the INTEREURO project. Although our approach has important advantages, we extensively discuss several challenges we faced as well as the procedures we developed in order to deal with these