Spatio-temporal variability assessment of Cape cormorant (Phalacrocorax Capensis) and bank cormorant (Phalacrocorax neglectus) diets at four Namibian Islands

Abstract

Seabirds form a fundamental component of the marine ecosystem and are excellent indicators of the health of marine environments. This study examined temporal, spatial and inter-annual variability of bank cormorant and Cape cormorant diets on four islands in Namibia from 2003 to 2017. Data from 2003 to 2014 was regarded as old data and data from 2015 to 2017 as recent data. Seabirds are declining drastically owing to scarcity of food and competition from commercial fisheries for prey species previously exploited. The seabird species under study feed on fish that are commercially as well as ecologically important such as sardines and bearded gobies. Both seabird species are listed as endangered owing to the decline of key colonies in Namibia and South Africa as a result of various threats such as diseases, lack of food, overfishing and commercial fisheries, among other factors. Diet regimes of the bank and Cape cormorants need to be properly understood in order to understand the functionality of the Benguela Current Large Marine Ecosystem (BCLME) as these seabirds are endemic to the BCLME. The research findings from the study can assist fisheries managers and scientists to better manage fisheries resources and make informed decisions as well as providing recommendations on marine protected areas, closure of fishing areas and nursery grounds (sanctuaries) for juvenile fish. The results of the study will provide information on recruitment areas of some prey items that are commercially important such as rock lobsters, as bank cormorants feed on juvenile rock lobsters both in Namibia and South Africa. Fresh regurgitated pellets were collected in their natural setting at seabird colonies on the four study islands. The pellets were then dried, hand-crushed and sorted, and prey items were identified with the aid of identification manuals. The main prey species for bank cormorants on Ichaboe and Mercury Islands were gobies, while for the southern islands the main prey species were rock lobsters and other crustaceans in terms of frequency of occurrence. The main prey species for Cape cormorants on all islands were gobies. Possession Island had the fewest number of gobies and Mercury Island had the highest number of gobies in terms of prey species per pellet for Cape cormorants. The Independent Samples t-test was used to test for significant differences in the diet of both species between the seasons (summer and winter). The null hypothesis was rejected and the significant level was less than 0.001 for both bank and Cape cormorant diets between the seasons. To test for significant differences between the islands and between years, one-way analysis of variance (one-way ANOVA) was used. The mean squares between the groups were 18.663 and 0.229 within the groups, giving a significant difference at 0.05 level. There was a spatial variation of both cormorant species on the different islands as well as inter-annual variation and temporal/seasonal variation in the diets of the bank and Cape cormorants on the different islands in Namibia. Economically important species such as rock lobster form the main part of prey species off Lüderitz (Penguin Island) and south of Lüderitz (Possession Island) for bank cormorants. To ensure the conservation of bank and Cape cormorants to avoid extinction, interventions from fisheries managers are needed such as a multi-species management approach and the introduction and promotion of MPAs and EBSAs. It is recommended that Marine Protected Areas within the seabirds’ feeding range need to be protected and monitored for illegal fishing and susceptible overfishing of recruitment stock.