StoX applied to cod and haddock data from the Barents Sea NOR-RUS ecosystem cruise in autumn - Swept area abundance, length and weight at age 2004-2017
MetadataShow full item record
North East Arctic cod and haddock are the largest stocks of these species in the world. Data from the Barents Sea NOR-RUS ecosystem cruise (BESS) in autumn is used as input to the assessment of these species. Here, we used the StoX software to recalculate the indices from the survey. First, we stratified the area covered by BESS into nine strata based on average cod and haddock density and modified this strata system by year to account for year to year variation in spatial coverage and survey design). We then used these strata systems to estimate swept area abundance indices, weights and lengths at age. We evaluated the quality of the indices based on uncertainty estimates and internal consistency. Coefficient of Variation (CV) from bootstrap runs was used as a measure of uncertainty. Low CV indicates that the number of samples are adequate relative to the abundance and patchiness of the targeted species. Consistency is the relationship between the abundance of a cohort at age a in year y with the abundance of the same cohort later at older ages. The indices for a cohort should decline over time due to natural and fishing mortality (assuming adequate size/age dependent fishing efficiency correction and adequate spatial coverage of all ages). The correlation between the log abundance of a cohort at age a in year y with log abundance of the same cohort at age a+1 (of ages used in tuning) in year y+1 were ≥0.73 for cod and ≥0.74 for haddock. The CVs were much higher for haddock than for cod, still internal consistencies were about similar for the two species. The CVs increased with age, especially for cod. There appear to be problems with haddock sampling, demonstrated by the very high CVs especially in 2016, a year with lack of coverage in the south-east, where most of the haddock was distributed. We recommend keeping the survey design of BESS the same every year. If changes are made to the design, sampling or gear, these changes should first be carefully considered and then documented. Survey indices for haddock could probably be improved by using a denser station grid in the main haddock distribution area, this will require more survey days. “Holes” and inadequate spatial extent of the survey strongly reduces the quality of the data, are difficult to adjust for and should be avoided.