Scallops were the third most valuable wild-caught marine animals in the United Kingdom (UK) in 2018, with a first-sale value of £69.7 million. Despite the high relative and absolute economic value of scallop fisheries in the UK, the majority are not managed based on quantitative scientific evidence from stock assessments which risks them becoming overfished and unsustainable. Scallops were also the third most valuable wild-caught marine animals in Wales, at a first-sale value of £2.4 million. Despite this Welsh scallop first-sale value being approximately 3.4 % of the value of the wider UK fishery, the fishery is still relatively economically important to Wales as the third most valuable. In addition, landings of scallops in to Wales have been decreasing since 2012 which highlights that greater scientific evidence is required to support the sustainable management of this relatively economically important natural resource. The existing management tools in Welsh waters are not linked to evidence of scallop stock sizes, or any other measure of scallop stock status. In addition to these arguments for sustainably managing the scallop populations, scallop dredging is considered to have a negative impact on the wider ecosystem and therefore it is important to gain a greater understanding of the negative effects of scallop dredging so that these may be better managed.
The aims of this study were to implement techniques to estimate two valuable pieces of evidence which could help support sustainable management of king scallops (Pecten maximus) in Wales. These pieces of evidence were; (1) estimates of absolute stock sizes and (2) the effect of repeatedly applying fishing effort to an area on the target species, the wider environment and fuel efficiency. Multiple historical reconstruction stock assessment models and a spatial depletion model were used to directly estimate absolute size of stocks or populations, and the catch efficiencies of multiple commercial vessels were quantified with a view towards scaling catch rates to abundance using catch efficiency in the future. The three historical reconstruction stock assessment models varied by estimated stock structure and were age-, length- and un- structured, where unstructured models are more commonly known as surplus-production or biomass dynamics models. The effects of repeatedly fishing small areas on environmental fishing efficiency and fuel efficiency were investigated through simulations and empirical data.
The estimated catch efficiencies of five commercial scallop vessels ranged from 0.13 to 0.62, which demonstrated high variability in catch efficiencies between the vessels and between estimates for the same vessel. This indicated that catch efficiencies can vary considerably between scallop dredgers and catch rates should be not be scaled to estimates of abundance using catch efficiency until greater understanding of catch efficiencies is achieved. Scallop density was found to vary considerably over small spatial scales (25 to 59 commercially sized scallops per 100 m2) and was not linked to sediment type. This reinforces the need for fishery-independent surveys to determine fine spatial scale fluctuations in scallop densities. Catch efficiency was also shown to be important when understanding the environmental impact of scallop dredging relative to catch as areas were repeatedly fished. In particular, vessels with a catch efficiency higher than the benthic depletion rate would cause a greater environmental impact relative to their catch as small areas are continued to be fished. This insight could be used to evaluate the trade-off between quantity of catch and environmental impacts of fishing and used to determine an effort threshold for vessels of particular catch efficiencies that could be used in a rotational management strategy.
The size of a king scallop stock in Wales was estimated by three different historical reconstruction stock assessment models and collectively these models also estimated a wide range of useful rates, states and parameters for scallop fisheries and life history including annual fishing mortality rate, recruitment, selectivity, catch efficiencies, maximum sustainable yield and more. In addition to these rates, states and parameters being directly useful for future analyses of this fishery, they would also be useful as prior distributions in stock assessments of other king scallop fisheries. Furthermore, it was shown that the unstructured model produced similar outputs (estimates of stock size and biological reference points) to the age-structured model which both agreed and disagreed with other studies.
These key pieces of evidence allowed the proposal of strategies to attempt to manage the Welsh scallop fishery sustainably. Two proposed strategies to reduce fishing mortality included imposing catch limits and by setting effort limits. These strategies could be applied to large regions of Welsh waters, relatively small areas or as limits on individual vessels. The primary recommended strategy is to use rotational management of small areas combined with effort limits assigned to vessels based on knowledge of catch efficiencies.