Ichnology applied to the study of Cookiecutter shark (Isistius brasiliensis) biogeography in the Gulf of Guinea

Abstract

Scarce bycatches of Cookiecutter Shark (Isistius brasiliensis) is the reason why this species is easier studied through the marks they leave on their prey rather than by direct observation of the shark itself. However, scientific studies that have used ichnology to remedy this lack of knowledge have been probably based on mistaken assumptions transmitted among previous authors without due verification. Despite identifying the problem almost twenty years ago, the authors of this paper were unable to obtain a sufficient number of Cookiecutter Shark specimens until now in order to confirm their hypothesis. This work provides a set of biometrics of Cookiecutter Shark specimens among which worth highlighting is the measure of the lower jaw width (LJW), which was absent in the relevant literature. This parameter is crucial for correcting the linear regression between total length and mouth width on which earlier biogeographical works were based. The new linear regression obtained using LJW was later applied to the bites observed on Swordfish caught in the Gulf of Guinea in 1996. The results show a probability of 0.50 for Cookiecutter Shark attack on Swordfish in the study area (29.4% of Swordfish bitten), and a positive correlation between predator and prey sizes. The lengths of Cookiecutter Shark inferred from bite size on sampled Swordfish revealed a certain geographical disaggregation in the area, with older individuals, probably females, concentrating in waters near the mouth of the Congo River.