Benchmark Workshop on North Sea and Celtic Sea stocks (WKNSCS​ 2022)​

The list of authors and the volume/issue numbers were

updated in February 2023.

A benchmark process was conducted for six stocks in the North Sea and Celtic Sea area with the goal of determining appropriate data inputs for assessing status, estimation of reference points, and short-term projection which could form the basis of advice for fishing prospects. The benchmark process consisted of two meetings separated by about three months: the first was a data inputs review and the second was a model assumptions and outputs review.

Benchmarked evaluation methods were produced for:

1) North Sea, Skagerrak, Malin Shelf Haddock (had.27.46a20)

2) Irish Sea cod (cod.27.7a)

3) North Sea plaice (ple.27.4)

4) Celtic Sea plaice (ple.27.7fg)

5) North of Scotland herring (her.6aN)

6) North of Ireland, West of Scotland herring (her.6aS7bc)

North Sea Haddock is a category 1 assessment using the standard ICES Stock Assessment Model (SAM):

The previous model for this stock was replaced by SAM in this benchmark primarily because the originator of the previous model is about to retire and a SAM approach is better under-stood by a wider swath of the ICES community presently. Furthermore, because the old model is incompatible with MS-Windows 12 and it will not be updated, it was deemed desirebale to switch to a SAM assessment. The SAM model fitting for this stock showed good fitting diag-nostics and the approach was easily accepted as the benchmarked approach. Haddock are notiously sporadic recruiters and this makes the choice of stock-recruit relationship for deter-mining the reference points difficult but this is well know feature of this stock and other had-dock stocks and did not preclude the estimation of reference points.

Irish Sea cod was assessed as a category 1 stock that used the Stock-Synthesis 3 model ap-proach:

Irish sea cod was previously classified as a category 3 stock. It was proposed as a category 1 assessment for the present benchmark and used the highly adaptable stock-synthetis modelling approach. The main issues for this assessment are less about the model approach than about data and a stock structure. The stock has not experienced many commercial fishery removals for a number of years while a recreational fishery along the English coast has been roughly quantified. The productivity dynamics of the stock seem largely extrinsically driven (i.e. spawner biomass influence on recruitment is the not the primary determinant of stock growth presently), at least during the current low stock size period. A post benchmark process to de-termine an ecological/climate conditioned sustainable fishing mortality (which has become termed Feco) was consquently calculated for this stock. The next benchmark for this stock should consider recreational catches from across the stock area and should revisit the definition of what area and data constitute information pertinent for this stock.

North Sea plaice is a category 1 assessment using SAM:

This assessment combined survey indices to cover the whole stock area well. Age dependent natural mortality was implemented based on literature methods of producing mortality estimates for species with plaice life history. The model fittings with time variying and invariante M produced similar fitting diagnostics however. Discarding rates of plaice from this stock has historically been important and assumptions about discarding have important impact on the estimates of the reference points.

Celtic Sea plaice is a category 1 assessment using SAM:

There were considerable difficulties fitting SAM to the data for this stock. The accepted bench-mark model was subsequently rejected owing to poor fit diagnostics and an ICES category 3 methods was proposed as the way forward for producting advice.

Herring North and West of Scotland and Herring in Northwest of Ireland and West of Scotland were assessed using a category 3, method 2.2 CHR rule:

Early in the data meeting it was decided to split these into two stocks as genetic, survey and migratory evidence suggests that there are at least two major components in this stock complex.

A category 1 SAM assessment was attempted for the North of Scotland stock but it was too sensitive to the inclusion of particular survey indices combined with lack of strong justification for choosing particular survey series over others. Therefore the a category 1 SAM approach was rejected in favour of the ICES category 3 CHR rule which is based on life history an opti-mal length. The M/k ratio (natural mortality M and von Bertalanffy growth parameter k) de-fault of 1.5 for the method was modified to reflect actual values calculated from the data. This method adjusts catch in the recent period up or down depending on the state of the stock im-plied by the methods. Uncertainties remain in how split catch and survey data between this stock and the herring from western Scotland and Ireland and this will warrant future work.