Spatial, seasonal, interannual variability and long-term trends in the diet of Cape fur seals along the Namibian coast (1994 – 2018)
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Date
2022
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University of Namibia
Abstract
To fully understand how the Cape fur seal population responds to changing ecological and environmental conditions, and be able to comprehend their functional roles in the ecosystem, a thorough understanding of their diet composition, seasonality, interannual variation and long-term trends, is necessary. The diet of the Namibian Cape fur seal population was studied from prey remnants recovered across a 25-year time-series (1994 – 2018) of monthly scat samples (n = 1270) collected at various colonies situated along the entire Namibian coastline. Colonies in close proximity had similar prey matrices, and all colony samples were grouped into four geographical regions, namely the Far North (FN), Central North (CN), Central South (CS) and Far South (FS) regions.
The study found that bearded goby Sufflogobius bibarbatus was the most abundant prey species at all four regions. This was followed by Cape horse mackerel Trachurus capensis in northern Namibia (CN and FN) and the lanternfish Lampanyctodes hectoris in southern Namibia (CS and FS). Cape hake Merluccius capensis was the third most abundant prey species in three regions, but negligible in the Far North. All the other prey species combined contributed less than 7 % of the total numerical diet composition. The monthly contribution of the three main prey species varied in all regions, but, bearded goby was dominant during all months in the Far North and Far South regions. The study indicated that the bearded goby abundances were less variable in relation to other species in all the regions. However, the bearded goby seemed to be the least preferred prey. This
is because when Cape horse mackerel or lanternfish were available, the Cape fur seals readily switched and fed more on those than the more abundant bearded goby. There were significant interannual variations in the contribution of the first two most important prey species in each region, being negatively correlated to each other (r = -0.84, n = 10, P < 0.002; r = -0.62, n = 25, P < 0.001) for bearded goby and Cape horse mackerel in the FN and CN respectively, (r = -0.52, n = 25, P < 0.007; r = -0.63, n = 22, P < 0.002) for bearded goby and lanternfish in the CS and FS regions, respectively. The spatial, seasonal andlong-term variations in the contributions of the main prey species were related to the geographical distribution, abundances as well as the calorific values of each prey species (the bearded goby has the least calorific content of the considered species). There was a long-term increase in the proportion of the bearded goby in the diet of the Cape fur seals
in the Central North and Far South regions (t = 3.21, n = 10, p < 0.001; t = 4.06, n = 22, P = 0.001), an increase in Cape horse mackerel in the Far North region (t = -5.01, n = 10, P < 0.001) and a decline in lanternfish (t = -2.19, n = 22, P = 0.04) in the Far South region. Lanternfish and bearded goby are currently not exploited in Namibia. With minimal pressure from fishing, their variability in the diet is thus likely related to their natural variability in abundance, with possible alterations due to changing environmental conditions affecting their abundance directly or their availability to the fur seals through depth distribution. The study indicated that the abundance of the lanternfish declinedwhereas that of the bearded goby increased in southern Namibia (CS and FS). Therefore, to understand the decline in lanternfish abundance in the Cape fur seal diet in southern Namibia, the study further investigated the link between its discrete monthly and long term diet contribution and to the coastal upwelling index and sea surface temperature recorded across the same timeframe in the same area. There was a significant positive correlation between the discrete monthly lanternfish abundance and coastal upwelling in
the Central South region (r = 0.41, n = 90, P < 0.001) and a significant negative correlation between the discrete monthly lanternfish abundance and in situ SSTs in both the Central South (r = -0.27, n = 300, P < 0.001) and Far South region (r = -0.27, n = 300, P < 0.001).
The long-term monthly lanternfish abundances showed significant correlations with the SSTs in the Far South region (r = -0.60, n = 12, P = 0.04). This was the first attempt to relate the abundance of the lanternfish in the diet of the Cape fur seal to fluctuations in the coastal upwelling and sea surface temperature in Namibia. These correlations provided a view of how climate variability affected the diet of higher trophic levels. Furthermore, they revealed that the diet composition of Cape fur seals inferred from scat sampling do not only help us understand their dietary composition and variations over time, but also contribute to better understanding the underlying causes of abundance fluctuations - especially for species that are currently commercially unexploited (and thus not actively assessed and/or monitored in Namibia). The knowledge assimilated by this study therefore helps in better understanding the relative importance of prey species, especially the non-commercially important species to the diet of a top predator in a functional ecosystem, as well as insights into the relationships that these species have
with environmental variables such as coastal upwelling and sea surface temperature.
Description
A research dissertation submitted in fulfilment of the requirements for the Degree of Master of Science (Fisheries and Aquatic Sciences)
Keywords
Diet, Cape fur seals, Namibian coast