A mechanistic framework to inform the spatial management of conflicting fisheries and top predators

Impact of disturbance on common scoter carrying capacity based on an energetic model

Shallow coastal zones are intensely used by humans but simultaneously are biodiversity hotspots, with a crucial role in the life cycle of many marine species. The competition for food or space between humans and protected wildlife intensifies under pressure of an increased need for marine resources for human consumption. For successful management it is important to establish the key components driving such conflicts of interest. Here we focus on the protected common scoter (Melanitta nigra), a sea duck wintering in coastal habitats that are rich in food, but also among the most disturbed marine systems worldwide. Due to the scoters' shyness disturbance impacts the birds' ability to forage and poses a conflict for balancing bird conservation and economics, including a fishery on its main bivalve prey Spisula subtruncata. In this study, we use an energy budget model to quantify the consequences of depth, currents and disturbance on scoter energetics and carrying capacity. Energetics were described using physical parameters and field data on food availability and disturbance. Results reveal non-linear relationships and a threshold value for when a scoter can no longer maintain its energy balance. This is caused by limited foraging time, rather than food availability. From a conservation perspective, this implies that a precautionary principle should be used, because there will be no warning when an area becomes unsuitable. In addition, the model was applied to study the effects of disturbance from different kinds of shipping in a coastal area of the North Sea, north of The Netherlands. Cargo shipping has the largest impact on the carrying capacity, where there is spatial overlap of prey and an intensively used shipping lane. In other prey distribution situations shrimp vessels may cause most disturbance. Spisula-Ensis fisheries did not limit the potential carrying capacity due to the limited catches and number of fishing trips. Scoter protection should be aimed at flexible spatial management and on only those vessel types above a Spisula bed with a large number of trips, and above all should work from a precautionary principle given the critical thresholds for scoter presence.

Foraging range scales with colony size in high-latitude seabirds

Density-dependent prey depletion around breeding colonies has long been considered an important factor controlling the population dynamics of colonial animals.^1-4 Ashmole proposed that as seabird colony size increases, intraspecific competition leads to declines in reproductive success, as breeding adults must spend more time and energy to find prey farther from the colony.¹ Seabird colony size often varies over several orders of magnitude within the same species and can include millions of individuals per colony.^5,6 As such, colony size likely plays an important role in determining the individual behavior of its members and how the colony interacts with the surrounding environment.⁶ Using tracking data from murres (Uria spp.), the world's most densely breeding seabirds, we show that the distribution of foraging-trip distances scales to colony size^0.33 during the chick-rearing stage, consistent with Ashmole's halo theory.^1,2 This pattern occurred across colonies varying in size over three orders of magnitude and distributed throughout the North Atlantic region. The strong relationship between colony size and foraging range means that the foraging areas of some colonial species can be estimated from colony sizes, which is more practical to measure over a large geographic scale. Two-thirds of the North Atlantic murre population breed at the 16 largest colonies; by extrapolating the predicted foraging ranges to sites without tracking data, we show that only two of these large colonies have significant coverage as marine protected areas. Our results are an important example of how theoretical models, in this case, Ashmole's version of central-place-foraging theory, can be applied to inform conservation and management in colonial breeding species.

COVID-19 lockdown reveals tourists as seabird guardians

The widespread lockdowns put in place to limit the spread of the new coronavirus disease (COVID-19) offers a rare opportunity in understanding how human presence influence ecosystems. Using data from long-term seabird monitoring, we reveal a previously concealed guarding effect by tourist groups on an iconic seabird colony in the Baltic Sea. The absence of tourists in 2020 lead to a sevenfold increase in presence of white-tailed eagles Haliaeetus albicilla, a sevenfold increase in their disturbance of breeding common murres Uria aalge and causing 26% lower murre productivity than the long-term average. Eagles did not prey on murres, but their frequent disturbances delayed egg laying and facilitated egg predation from herring gulls Larus argentatus and hooded crows Corvus cornix. Based on our findings, we suggest that human presence could be used as a strategic measure in guarding seabird colonies, and that a social-ecological systems perspective is vital for long-term success in protected area management.