3/2/2015 ICES Benchmark Workshop on West of Scotland herring (including Celtic Sea, VIIg,j herring) (WKWEST 2015) Monday 2 February – Friday 6 February 2015, Dublin, Ireland ICES Chair: Richard Nash, Norway External Chair: Steve Cadrin, USA Invited Experts: Tim Miller, US and Gary Melvin, Canada Professional secretary: Anne Cooper The problems to be faced by the Benchmarking process. Herring to the west of the British Isles 1. The herring stock in VIaN was perceived to be at a low but stable level and VIaS was perceived to be declining and had a zero TAC advice. 2. The quality of the assessments were increasingly being questioned both from a scientific and stakeholder perspective. 3. Relatively recent scientific research indicated that the stocks which spawn in the ICES management areas VIaN and VIaS mix, to an unknown degree on the Malin Shelf and there most probably was an unknown degree of mixed catches. 4. An acoustic survey to determine the abundance of herring in the area was enhanced to ensure both stocks were covered and this survey indicated a decline in the abundance of herring in VIa. The VIaN geographical survey was also showing a (noisy) signal of decline but that was not mirrored in the SSB trajectory in the old VIaN assessment. 5. In regard to determining the sizes of the two stocks from the combined Malin Shelf acoustic survey the ability of being able to identify each stock was essential. 6. Recent re-examinations of methodology for identifying individuals from each stock caught in the surveys or even catches, where the stocks may be mixed, Management areas Spawning Feeding do not appear to have an acceptable level of precision. And then Celtic Sea VIIg,j was added because: Status quo and management: i. Recent Benchmark assessment routine left very serious ‘retrospective patterns’ in the outputs making the a. Division VIa consists of two management areas which have different management objectives. There assessment questionable. is a need to provide advice for both. ii. Explore other assessment models which may be more b. Over the course of the time that these two stocks appropriate for this stock have been subject to assessment one or other has iii. Address reference points once a new assessment is been dominant. agreed upon. c. Data to date suggests that one of the stocks is much smaller than the other, therefore there is a need to ensure the smaller stock is not over exploited. 1
3/2/2015 Commercial landings/catches for Area VIaN and VIaS, VIIg,j Herring acoustic surveys undertaken around the west, north and east of the British Isles (mature autumn spawners) (2013) Biomass of mature autumn spawning herring in June – July 2013 Scottish Ground fish Survey – Quarter 1 Herring acoustic surveys undertaken around the west, north and east of the British Isles (immature autumn spawners) Scottish Mackerel Recruit Survey – Quarter 4 D7 D9 E1 E3 E5 E7 52 Vb 51 50 49 60 48 47 46 45 58 VIa 44 43 42 41 Latitude 56 40 39 38 37 54 Biomass of VIIa 36 VIIb 35 immature 34 33 52 autumn 32 31 VIIg spawning 30 VIIj 29 50 28 herring in VIIe 27 VIIh June – July -12 -10 -8 -6 -4 -2 Index areas: 1 whole of VIa, 2 VIaN, 3 VIaS. Longitude 2013 Haul positions 1986-2010 Survey changed from ICES rectangle based (similar to North Sea IBTS) to a semi-stratified random design. Similar station coverage though. Treat as two 1 2 different surveys. Internal consistency: Can the survey follow year classes through successive years? Identification of individuals to stocks Ecosystem drivers – a changing environment Body Morphometry New samples – 2014/15 Variability in surface temperature 2004/05 baseline - WESTHER of coastal waters around Scotland. Long-term (1900-2006) variability in Otolith Shape oceanic temperatures to the north of Scotland and east of Faroe (including the Northern Hemisphere (NH) (source NOAA). Dashed lines: 1980-2006 (0.24 ° C per decade) 1900-2006 (0.04 ° C per decade) North of Scotland, east of Faroe, west of Scotland and S01 - S06 - CELTIC IRISH the northern North Sea. CELTIC IRISH VIaN VIaS SEA SEA SEA SEA VIaN VIaS CELTIC IRL 141 SEA 5 0 4 Donegal1 0.04 0.04 0.13 0.79 IRISH SEA 4 162 1 0 IRL Donegal2 0.04 0.02 0.14 0.80 Salinity (1970-2006) Temperature (1970-2006) VIaN 0 SCO S08 4 231 22 Hebrides 0.00 0.05 0.48 0.47 VIaS 3 6 12 348 SCO S10 Cape Wrath 0.01 0.05 0.48 0.46 2
3/2/2015 Natural mortality (M) issues and predation Use of information from the industry In 2014 the fishing industry encountered and documented a large aggregation of herring (dimensions 2.7 by 1km). Using density estimates from other surveys of aggregations and reasonable potential packing densities and mean weights, an ad hoc and very subjective estimate of 27,000t was generated. This would be very substantial part of the assumed biomass at the time. Again in 2015 large aggregations of herring were observed and recorded in the area. The information as collected and reported cannot be used in a quantitative matter to evaluate the stock status, however, it does confirm that relatively large aggregations of Estimates of natural mortality by age fish are being observed during the spawning season in the region, their abundance may be increasing and that these biomass observations should be investigated. Proceeding to an assessment VIa, VIIb,c assessment – looking for quality Three different models explored: 1. Separable VPA, 2. 1. Neither VIaN nor VIaS provided reliably stable assessments. Integrated Catch Analysis (ICA) and 3. State-space stock 2. A combined VIa, VIIb,c assessment was reasonably assessment model (SAM). stable (looking for small and random variations in the residuals to 3. Work on the assumption of mass balance i.e. the model fits and small and random retrospective patterns) assessment of both together was a simple summation of both 4. Initial hope was either an assessment of either VIaN or VIaS was possible and the difference between the two was the size of the missing stock 5. An alternative was a method of determining the relative sizes of each stock so the total abundance could be partitioned between the two stocks. 6. Methods of achieving 5 were explored. Examples of bubble plots of residuals from Separable VPA and SAM The SAM model approach was chosen as the best Where does this leave us with area VIa, VIIb,c and the two All four surveys (1. Acoustic survey for VIaN, and 2. Malin Shelf series, management areas. plus 3. 1 st Quarter Bottom trawl survey and 3. 3 rd Quarter Bottom trawl survey) were included. 1. We can determine the biomass of herring that can be There were no statistical reasons for removing any of the survey indices from the assessment. found to the west of the British Isles (in Area VIa, VIIb,c). 2. Of that biomass we can not (at present) reliably estimate the proportions that can be considered as spawning in each of the two ICES management areas i.e. VIaN and VIaS, VIIb,c. The Benchmark will Report to ICES and the HAWG Final assessment using 2015 updated data will be undertaken at the HAWG HAWG will present management recommendations through the standard ICES channels 3
3/2/2015 Natural mortality issues – again/as well Celtic Sea VIIg,j (short summary) Last Benchmark switched from ICA to SAM. Problem with the A considerable amount of work was carried out in 2014 and retrospectives when taken to assessment in 2014. average modelled North Sea mortality rates for the years 1963 to 2010 were used. The North Sea herring underwent similar stock Various models tested and ASAP (A Stock Assessment Program) is an trajectories over time to this stock, and provided the basis for age-structure stock assessment modeling program, chosen based on analytical criteria that assesses the level of retrospective patterns (rho). choosing these values. The overall choice was based on the following preferences: The new data varied mortality with age but did not try and ‘guess’ the inter-annual variation in mortality. Natural mortality schedule used in 2014 (average of 1963 to 2010) 1 2 3 4 5 6 7 8 9 0.793 0.377 0.351 0.322 0.312 0.307 0.301 0.301 0.301 Natural mortality schedule used in 2015 (average of 1974-2013) 1 2 3 4 5 6 7 8 9 0.706 0.353 0.280 0.255 0.240 0.226 0.217 0.217 0.217 Comparison of 2014 model and settings versus 2015 Retrospective patterns Finally: model and settings (2015 model) WKWEST suggests little change, or none at all in management reference points relative to 2013 and 2014 Blim Bpa MSY B trigger Management trigger Flim Fpa Fmanagement The one major exception being a lowering of F MSY from the un-realistically high level of 0.37 suggested in 2014. 4
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