Abstract
Underwater Television (UWTV) surveys provide fishery-independent stock size estimations of the Norway lobster (Nephrops norvegicus), based directly on burrow counting using the survey assumption of “one animal = one burrow”. However, stock size may be uncertain depending on true rates of burrow occupation. For the first time, 3055 video transects carried out in several Functional Units (FUs) around Ireland were used to investigate this uncertainty. This paper deals with the discrimination of burrow emergence and door-keeping diel behaviour in Nephrops norvegicus, which is one of the most commercially important fisheries in Europe. Comparisons of burrow densities with densities of visible animals engaged in door-keeping (i.e. animals waiting at the tunnel entrance) behaviour and animals in full emergence, were analysed at time windows of expected maximum population emergence. Timing of maximum emergence was determined using wave-form analysis and GAM modelling. The results showed an average level of 1 visible Nephrops individual per 10 burrow systems, depending on sampling time and depth. This calls into question the current burrow occupancy assumption which may not hold true in all FUs. This is discussed in relation to limitations of sampling methodologies and new autonomous robotic technological solutions for monitoring.
Highlights
The Norway lobster, Nephrops norvegicus (L.), is one of the most commercially important fisheries in Ireland and Europe[1]
The diel rhythm of burrow emergence is more complex than previously thought and it can be subdivided in three different phases11,15 : Full emergence, full retraction and an intermediate period in which individuals wait at the burrow entrance
Door-keeping behaviour had some defined pattern with crepuscular peaks coinciding with full emergence only on the upper shelf (15–50 m depth) and the shelf-break (101–160 m depth)
Summary
The Norway lobster, Nephrops norvegicus (L.), is one of the most commercially important fisheries in Ireland and Europe[1]. Peaks in trawl Catch Per Unit Effort (CPUE) shift in timing with increasing fishing depth[11,12,13]: from full night to dusk- dawn transitions, going from upper to middle-lower shelves, to be fully diurnal (i.e. at midday) on upper and middle slopes This indicates that the species sets its timing of burrow emergence upon a maximum illumination threshold that varies on the depth axis, based on the differential penetration of light as the sun progresses through its diurnal trajectory[10,14]. Of the Sea Working Group on Nephrops Surveys (WGNEPS)[1,22,23] This more direct (i.e. image-based) method of assessment counts burrow systems, based on their characteristic structural features The burrow system counts can be used as a relative or absolute index for determination of Nephrops’ stock status and together with catch data can provide a Harvest Rate (HR; catch in numbers/burrow numbers)[28, 29]
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