Fighting over burrows: the emergence of dominance hierarchies in the Norway lobster (Nephrops norvegicus)

Abstract

Animals fight over resources such as mating partners, territory, food or shelter and repeated contests lead to stable social hierarchies in different phyla. The group dynamics of hierarchy formation are not characterized in the Norway lobster (). Lobsters spend most of the day in burrows and forage outside of them according to a diel (i.e. 24 h-based) activity rhythm. Here, we use a linear and generalized mixed model approach to analyse, in seven groups of four male lobsters, the formation of dominance hierarchies and rank-related changes in burrowing behaviour. We show that hierarchies emerge within 1-3 days and increase in steepness over a period of 5 days, while rank changes and number of fights gradually decrease over a 5-day period. The rank position determined by open area fights predicts the outcome of fights over burrows, the time spent in burrows, and the locomotor activity levels. Dominant lobsters are more likely to evict subordinate lobsters from their burrows and are more successful in defending their own burrows. They spend more time in burrows and display lower levels of locomotor activity outside the burrow. Lobsters do not change their diel activity rhythms as a result of a change in rank, and all tested individuals showed higher activity at night and dusk compared with dawn and daytime. We discuss how behavioural changes in burrowing behaviour could lead to rank-related benefits such as reduced exposure to predators and energy savings.

Burrow emergence rhythms of Nephrops norvegicus by UWTV and surveying biases

“…Field observations indicate a more complex behavioural situation where single individuals can inhabit a single or complex burrow system with a variable number of entrances depending on local population densities 30 . At the same time, laboratory studies on aggressive hierarchy show that dominant individuals attempt to evict the subordinates to conquer their burrows nearby 9 . Even in periods of peak emergence it is possible that not all individuals are visible at the surface.…”

Density-dependent growth in ‘catch-and-wait’ fisheries has implications for fisheries management and Marine Protected Areas

“…slower-growing males, which were affected most by density and ‘superior’ faster-growing individuals. Sbragaglia et al (2017) looked at aggressive interactions and the formation of dominance hierarchies in Nephrops and showed that these can be set up in less than 5 days. Dominant individuals also spent more time in their burrows (Sbragaglia et al 2017), implying that they required less time to forage, which presumably enables them to divert more energy towards growth.…”

“…The potential for negative density-dependent effects as stock sizes increase is unknown, but there are reasons to suspect such effects in this lobster’s populations. Nephrops is a mud-dweller, sheltering within burrows (Lauria et al 2015; Sbragaglia et al 2017) and mud habitats may be a limiting resource for this species. A dome-shaped response has been demonstrated between burrow density and habitat quality (expressed as the percentage silt plus clay of the sediment).…”

The potential of video imagery from worldwide cabled observatory networks to provide information supporting fish-stock and biodiversity assessment

“…European landings of Norway lobsters were around 44 000 tonnes valued at $360 million EUR in 2016 (EUROSTAT, ec.europa.eu/eurostat/web/fisheries/data/database). Norway lobsters dig and inhabit complex burrow systems in muddy habitats used for shelter and for territorial control, from which they emerge to find food (Sbragaglia et al, 2017). Burrow emergence patterns differ with relation to depth and time of the day (Aguzzi and Sardà, 2008): from nocturnal to crepuscular on upper and lower shelves to diurnal on slopes.…”

“…Emergence is modulated not only by the stage of the reproductive cycle but also by size and other more contingent ecological factors (e.g. the presence of predators or prey; Sbragaglia et al, 2017). Such modulation represents a behavioural mechanism that protects this commercially exploited population from trawling because when individuals are in their burrows they are inaccessible to trawling.…”