Benchmark Workshop on Sprat (WKSPRAT 2018): 5 – 9 November 2018, ICES HQ, Copenhagen, Denmark

The benchmark workshop on sprat (WKSPRAT) was held in ICES headquarters in Copenhagen from 5–9 November 2018. The benchmark process started in June 2017 with a 3 day data compilation meeting, also held at ICES. Three stocks were benchmarked: North Sea sprat, 3.a sprat and the Channel sprat. During the benchmark process, several arguments were put forward on the connection between the North Sea stock and the 3.a stock in Kattegat-Skagerrak.

It was therefore agreed to merge the two stocks and assess them as one stock assessment unit. Sprat in area 4 and 3a During the 2018 benchmark, enough evidence were presented to merge the sprat stocks in the Skagerrak-Kattegak and in the North Sea. The catch data and the indices of abundance from 3.a were included in the data from area 4.

Three surveys are carried out in the area (IBTS in Q1 and Q3 and HERAS survey) and were all used as tuning indices in the model. The IBTS indices were standardized using a delta-GAM approach: the inclusion of 3.a data increased the internal consistency between all age classes for all indices. The SMS model, previously used to assess the North Sea component, was used to assess the new combined stock.

The model year is resolved into 4 seasons, where season 1 corresponds to quarter 3 of the calender year. The term ‘season’ is hence used here to identify the model year, while the term ‘quarter’ refers to the calendar year. The final model formulation includes a power function for the age 0 catchability of IBTS Q1, a constant maturity ogive and the inclusion of the very few catches reported for season 4 (Q2) into season 1 (Q3) of the following year.

The new stock assessment shows a considerable improvement in the retrospective pattern, as well a better fitting to some ages of the IBTS surveys and catches (as indicated by survey and catch CVs). The stock reference points were revised following ICES standard guidelines. Blim was estimated as the breakpoint of the segmented stock recruitment relationship and Bpa was derived from Blim.

Short-term forecast settings were updated. The Management Strategy Evaluation (MSE) settings for the estimation of the Fcap were presented and discussed. Several scenarios were proposed to be tested and results will be evaluated during an ad-hoc MSE workshop that will take place on 11–12 December 2018.

Sprat in area 7d,e Not enough evidences were available to change the boundaries of the English Channel stock. The stock is a category 3, and the data available are still scarce. An acoustic survey (PELTIC) is carried out in the English part of area 7.e since 2013. In 2017, the survey has been extended to include also the French part of 7.e and in 2018 the Eastern Channel was surveyed as well.

Also, an IBTS index in Q1 is available for the Eastern Channel from 2007 onwards. The time series from the PELTIC survey has been used since 2017 to provide advice. At the benchmark, the updated time series was presented: the method used to produce the acoustic index was revised compared to previous years and it was changed to StoX (StoX, 2015).

Concerns were raised because of the uncertainty in the stock distribution, and the fact that the acoustic survey covers only the Western part of the English Channel. No analytical assessment seems to work so far: both a seasonal and non-seasonal SPiCt were attempted during the benchmark workshop, but the short time series in the acoustic index and the lack of any contrast in the data still does not allow the model to converge.

The group agreed to provide a seasonal advice based on an empirical method, i.e. to use both indices for trends, but only the acoustic survey for provision of advice. Two advice rules were proposed: the 1 over 2 rule, and a fixed F rule. Both options will be tested in an MSE framework and presented during the ad-hoc MSE workshop that will take place on 11–12 December 2018.