Static-gear fisheries using baited pots are an increasingly valuable component of the fishing industry in the Irish Sea. Target species include Common whelk (Buccinum undatum), Edible crab (Cancer pagurus), European lobster (Homarus gammarus), and Common prawn (Palaemon serratus). Together, the harvest of these four species from the Irish Sea area (ICES division VIIa) by British vessels totalled 7,938 t in 2019 and were valued at £14.67 million at the point of first sale. However, there is an absence of evidence-based management applied to the fisheries as a consequence of data-poor status in key areas including biological and ecological understanding of life-histories, fisheries-dependent information on catch and effort, and scientific assessment methodologies for stock assessments. This thesis aims to address specific knowledge-gaps important for the sustainable management of baited pot fisheries in the Irish Sea.
Despite its relatively small size, the common prawn fishery is an economically important seasonal fishery in areas such as Cardigan Bay, Wales. The stock is prosecuted by a regulated open access fishery with no biologically referenced harvest control rules. In response to fluctuations in landings the fishing industry have adopted a size-selective harvesting regime of 10 mm carapace length (CL). This thesis presents a baseline of evidence for this fishery, and pays particular attention to the biological implications of size-selective harvesting and reproductive biology. Peak spawning occurred during the spring, whilst functional maturity of females was estimated at a CL of 9.9 mm. The species is sexually dimorphic, with females attaining a greater size than males. Size-selective harvesting results in a sex bias in landings as even large mature males often fail to recruit into the fishery.
Whilst whelk fishing activity covers large geographical areas of the Irish Sea, whelk populations are known to be vulnerable to localised overexploitation, and despite the prominence of common whelk fisheries in the Irish Sea, they remain relatively unrestricted. This thesis evaluates fundamental biological parameters within an environmental context, focussing in particular on reproductive biology and growth. Analyses highlight that, at the time of sampling, the majority of populations were not adequately protected by technical measures with size-at-maturity estimates being greater than the applicable minimum conservation reference size. Using previously validated ageing techniques, size-at-age relationships are presented for Irish Sea populations. L∞ showed a significant negative linear relationship with sea temperature. Other parameters, including maximum growth rates and the age at which growth rate begins to decrease, showed clear trends with sea-bottom temperature but linear modelling failed to detect significant relationships.
Despite the growing call to utilise fishery-dependent data in lieu of scientific stock assessments for data-poor fisheries, the resolution of these data are often insufficient for accurate catch and effort analysis. By trialling an Enhanced Electronic Reporting System (EERS) in the ‘mixed’ edible crab and European lobster fisheries in the Irish Sea, which integrates gear-in gear-out and mobile technologies, fisheries-dependent data has been shown to be a viable method of assessment. Using generalised additive models to investigate a dataset of environmental and fishing activity data, non-linear predictors of commercial landings-per-unit-effort (LPUE) data are estimated, which can be used to ‘standardise’ data. These analyses and techniques are later discussed in terms of integrating standardised LPUE data into management of the Isle of Man edible crab fishery specifically, and baited-pot fisheries more generally.
Finally, population data are modelled to evaluate technical conservation measures in the European lobster fishery and common whelk fishery. Mark-recapture and size-structure data are combined to form an analysis and evaluation of the likely economic impacts to a change in minimum conservation reference size for European lobster in Isle of Man territorial waters. Size-selectivity of grading equipment trialled in the Isle of Man are assessed for common whelk fishery with reference to the earlier size-at-maturity estimates for the same population.
Separately, the data presented in this thesis address a range of knowledge-gaps in baited pot fisheries in the Irish Sea by focussing on specific data deficiencies. Together, they offer valuable improvements and insights into evidence-based fisheries science, conservation, and management in the region, and are useful references for management of baited-pot fisheries globally.