ICES WKPLE REPORT 2015
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Executive summary
Stock identification issues were examined for three of the four stocks: based on larval
drift, otolith characteristics, genetics, and tagging studies the Skagerrak stock was
evaluated to have strong connectivity to the North Sea plaice stock. North Sea plaice
are having extensive feeding migrations into Skagerrak. The likely magnitude of the
stock mix was considered sufficient to recommend and approve that plaice in the
North Sea and Skagerrak are combined and assessed as one stock although some
stock components in Skagerrak are considered resident. Given the size of the fishery
in Skagerrak (about one tenth of the North Sea) the addition of catches from Skagerrak to the North Sea assessment has little impact on the combined assessment. For the
combined North Sea and Skagerrak stock, future attempts should be made to include
the IBTS data since it seemed to improve the quality compared to the IBTS data
which excluded the Skagerrak region. In addition monitoring the Skagerrak proportion should be a high priority for the North Sea stock assessment and advice. This
should be done to avoid local depletion of the resident stock.
The stock entity of plaice in Subdivisions 21-23 and in Subdivisions 24-32 is less well
defined and available studies are inconclusive. WKPLE reviewed arguments to include 21-23 and 24-32 in one stock and had not sufficient arguments to suggest a deviation from the present perception of two stocks, the Kattegat-Belt stock (SD21-23)
and the Baltic stock (SD24-32). Examination of a combined 21-23 and 24-32 assessment was initiated but further work is required.
The stock assessment for the Eastern Channel plaice stock in Division VIId was improved with regard to input and assessment method. Discard estimates from 2006–
2013 are considered representative for the historic period and are included in the
assessment. The commercial cpue series from the Belgian beam trawler fleet was rejected as tuning fleet because of poor performance to track cohorts and concerns on
changes in fishing practices over time. Natural mortality by age group was estimated
and included in the analytical assessment. The previous accepted assessment model,
XSA, was rejected as a category 1 assessment model due to the lack of account of
discards. A statistical catch-at-age model including discard information, Aarts and
Poos (2009), was approved as the assessment model for this stock. Relevant future
work should include sensitivity estimates to the strong assumption of 90% discard
rates for age-1 over time since this appears to be the cause of the high amount of discards. The impact of size-based discards instead of the Aarts and Poos assumption
that it can be adequately tracked by age should also be considered.
Previous assessments of the plaice stock in Kattegat and the Belts (SD 21-23) were
qualitative and data limited stock approach were used for catch advice (category 3.2
stock). This benchmark reviewed and re-estimated a number of input parameters to
the assessment. Four surveys (NS-IBTS(SD21) 1st and 3rd quarter, BITS(SD21-23) 1st
and 4th quarter) were combined by use of a standardization into two survey indices
(1st quarter and 3rd–4th quarter). This reduced much of the noise in their performance
to track cohorts. Also likely noise in individual weights in stock and in maturity was
reduced by assuming a fixed age pattern for all years. Ageing difficulties is recognized to be one of the main causes for the noise in input data. Based on the two combined surveys an analytical age based assessment, SAM, was accepted and the stock
is therefore now assigned a category 1 stock. Future research should consider activi-
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ICES WKPLE REPORT 2015
ties involved in improving the efficiency and standardization of fishery data collection particularly on discards. Evaluation of length-specific survey indices highlighted
the potential benefit of models that could fit to length compositions directly. Further,
modelling approaches should explore possibilities to allow incorporation of historical
catch time-series. Since the sampling protocols have changed in recent years the assessment model should be developed so that variable (by year) catch estimation uncertainty can be accounted for.
Small and dispersed landings of plaice in the Baltic (Subdivision 24-32) prevent proper sampling and result in a noisy catch-at-age matrix. In addition high and variable
discard rates from fisheries targeting other species in the Baltic, i.e. discards without
any landings, impeded accurate discard estimates in the Baltic. Basis for stock status
continues therefore to be surveys conducted in 1st and 4th quarter. SAM modelling
with the surveys provides SSB estimates with high uncertainty but acceptable for a
trend based assessment. The computed SSB is considered for use in an indicative
assessment with DLS approach to base advice upon. Future research should consider
methods to improve discard estimates to deal with discards with zero landings. Also
landings estimates should be refined to ensure plaice and no other species are included. Given the relative lack of stock id studies for this area continued work should
explore stock structure and potential for a combined 21-23 and 24-31 assessment.
Considering the large proportion of discards and small landings more flexible modelling platforms should be evaluated to deal with size-based discards and variable observations errors over time.
WKPLE was not able to explore and define reference points for the Kattegat and Baltic stocks due to time constraints. The recent protocols on estimation procedures developed by WKMSYREF3 and WKLIFE4 both for stocks with a full analytical
assessment and for data limited stocks is expected to serve as appropriate objective
guidelines to derive on reference point estimates prior to the 2015 working group
meetings of WGBFAS.
Generic for all stock considered at WKPLE was concluded that given the high uncertainty in mix between areas and degree of connectivity continued work on stock identification should be conducted in order to be able to better quantify the
migrations/drift. Therefore explorations of alternative models that incorporate spatial
issues and capture length based dynamics (discards, selectivity, ageing problems,
migrations, drift) are also recommended.