Stock Assessment

Quantitative stock assessments are used to estimate the abundance (biomass) of a stock and to ascertain whether stock biomass is above or below a predefined reference point (i.e. whether it is underexploited, sustainably exploited or overexploited). In addition, the potential outcomes on the stock of various fishing mortality rates and management scenarios can also be modelled. In order to assess a stock at the correct level the biological boundaries of the stock must first be defined.
Examples of our approaches to stock assessment and research projects we are involved with include:

North Irish Sea Stock Assessment for king scallops (ICES WGSCALLOP)
Work on progressing a stock assessment for king scallops in the North Irish Sea is being undertaken as part of the International Council for Exploration of the Seas (ICES) Working Group on Scallop Stock Assessment (WGSCALLOP) for which Bangor Scientist Dr Isobel Bloor is co-chair and leads on this specific term of reference. The WGSCALLOP sub group meets virtually throughout the year as well as reporting at the annual in person working group meeting every October. Members have been looking at combining fisheries independent survey data from multiple jurisdictions across the Irish Sea for use in a stock assessment. In addition available fisheries dependent data from Vessel Monitoring Systems (VMS) and logbooks have been combined into standardised indices and are being trialled within various models (i.e. VAST, SPict) and standardised indexes.

Stock assessment for queen scallops in the Isle of Man territorial waters
An annual assessment of the queen scallop stock within Isle of Man territorial waters has been undertaken since 2012 using the Catch-Survey Analysis (CSA) method, first developed by Collie and Sissenwine (1983). The CSA method estimates stock size using abundance indices and has been advocated as a valuable method to support management advice where age data is not available as is the case for the Isle of Man’s queen scallop fishery. Within the stock assessment unit (Isle of Man’s territorial waters), the trend from the model output indicates that following five years of increasing biomass (2006-2010), total biomass has decreased during each of the subsequent eight years (2011–2018) before slight annual increases in each of the last 4 years (2019 – 2022) (2022: median estimated biomass of 3322 t).

Beam trawl survey data for crabs
Long-term datasets collected on the abundance, sex and size of brown crabs as bycatch in beam trawl surveys (BTS) undertaken within the Irish Sea for fish are being explored to assess potential as an index of abundance for stock assessment. In this instance a subset of this historical data set has been utilized to investigate the use of BTS data as a fishery independent survey method and its capabilities of detecting the trend of C. pagurus abundance in the North West Irish Sea. Utilizing a general additive model, trends indicate increasing overall abundance since 1995, with a peak in 2015 followed by a significant rapid decline in the past 5 years with a similar declining trend detectable in the Isle of Man landing per unit effort (LPUE) logbook time series. The identification of these trends and the utilization of such historic data presents a number of opportunities in the management and forecasting of fishery status in future. This work will be developed in collaboration with the wider scientific community through the International Council for Exploration of the Seas Working Group on crab to better understand these trends and any underlying drivers of C. pagurus abundance.