The survey results confirm the decline in stock numbers, and consequently in SSB, which was expected by ACFM last year.
The 1989-1991 year classes were strong and have supported the fishery in recent years. Since 1995 recruitment at age 3 has been low, except for the 1996 year class for which survey evidence is conflicting. The 1998 year class appears to be stronger than the previous ones, and the 1999 year class may also be so.
The 1999 fishery was still dominated by age 5 and older fish. However, in 2000 about half the Norwegian landings in numbers so far are comprised of the 1996 year class, which has just reached minimum landing size. This indicates that the older year classes may by now have been depleted.
An exploratory XSA gave results in line with these observations, but indicated a higher fishing mortality in recent year
than estimated previously. It now appears that the fishing mortality in recent years may be slightly above F
lim. The SSB
still appears to be above B
pa, but is expected to decline.
4 FUTURE CONSIDERATIONS AND ALTERNATIVE APPROACHES 4.1 Fleksibest
The Fleksibest model which is being developed as an alternative assessment method for the cod in particular, was presented to the WG at its last meeting (ICES 2000 - AFWG rep). Since then, an improved model for transferring mean growth to length distributions has been implemented, as well as, parallelisation of the code. Work is underway to include i.a. a broader range of objective functions with improved optimisation routines, as well as improved routines for post-processing and presentation of the results. The input data were updated with the current catch and survey data. A paper describing the method is now being prepared. Extensive exploration of the various model assumptions has been initiated. Some runs were presented to the WG, which show essentially the same trends in stock abundance and mortalities as the results presented to the last WG. The current plan is to use the model as an alternative assessment tool for cod at the next ordinary meeting of the Working Group.
4.2 Exploring stock abundance estimates using only survey data
In assessments where survey data are used to calibrate a VPA or a separable model, there sometimes appears to be a conflict between survey results and the magnitude and age structure of the catches. In some cases there may be doubts as to the correctness of the catch at age data. With respect to the NEA cod, this has been considered as a possible problem. However, for this stock the most conspicuous conflict is that the abundance according to the surveys fluctuates much more over time than indicated by the converged VPA. In order to explore the information about the stock abundance and age structure inherent in the survey data, it may be informative to attempt to estimate parameters in a stock model using only the survey data.
Since survey data are relative measures of the stock abundance, catchabilities are needed to translate survey indices to stock numbers. The usual assumption is that the a survey index I is related to the corresponding stock number N by a catchability q as: I = q*N. Commonly, catchabilities are assumed to vary with age and be constant over the years. Two alternative ways of estimating the catchabilities have been explored by this Working Group: By comparing the survey data with converged VPA stock numbers (Nakken WD #1 and #2) or by fitting a population model with separable fishing mortalities .
4.2.1 Calibration of survey indices using converged VPA
Two working document by Nakken was presented. In the first (WD#1) the survey indices found from the Norwegian bottom trawl survey in winter and the acoustic indices of spawning of spawning biomass were calibrated to yield estimates of stock numbers at age and spawning stock biomass. Calibrations were carried out using linear regressions on converged VPA stock numbers an associated index values. Since 1993, the area covered by the bottom trawl surveys has been larger than in previous years and before carrying out regressions the bottom trawl indices were adjusted to compensate for this. The calibration period for the bottom trawl indices was 1981-1992 and that for the spawning stock biomass index was 1985-1995. For later years, the observed indices were used directly together with the calibration results to yield stock estimates.
The results, which are presented in detail in the Working Document, show that
1. The survey estimates of stock numbers of ages 5,6 and 7+ for the years 1993 through 1998 are closer to the 1999 VPA estimates than are the annual estimates.
2. The survey estimates of spawning stock biomass for 1996,1997 and 1998 are much lower than those from the various recent WG assessments, while for 1999 the survey estimate is close to that from the assessment.
Another working document by Nakken (WD#2) commenting on the WG’s assessment of North-East Arctic cod in
autumn 1999 was also presented. The document, which was written to ACFM prior to the meeting of the committee in
autumn 1999 pointed out the discrepancies between the results of the annual assessments since 1995. The main point in
the document was that the stock numbers of 4 and 5 year olds in 1999 probably were overestimated by the XSA. By
comparing ratios between estimated stock numbers (1999 XSA) and corresponding swept area indices from Norwegian
− Stock numbers of 5 and 4 year olds as estimated by the WG seem to high as compared with swept area indices and swept area stock estimates.
− Survey results indicate that the 1998 and 1999 year classes are less abundant than the preceding year classes.
4.2.2 Fitting a population with separable fishing mortalities
In a WD by Skagen (WD#4), a simple separable population model was constructed on a spreadsheet, basically on the principles suggested by Cook (1995) and applied lately to the VIaN herring stock (ICES C.M.2000/ACFM:10). The fishing mortality was assumed to be separable, with a selection pattern modelled as a logistic function. Separable fishing mortalities were assumed for the period 1987 - 1999. For the previous years, a simple VPA, using the reported catches was made, initiated with the estimated stock numbers for 1987. For each survey, the catchabilities at ages 1, 2 and 3+ were taken as parameters, while the catchability was assumed to be constant for ages 3 and older. In order to scale the population, a constraint was applied that the total modelled catch in tonnes over the years 1987-1999 should equal the reported catch.
Altogether, the stock estimates were relatively consistent, and it is remarkable that the catch residuals in the later years were modest, except for the youngest ages. However, for the years before 1993, the modelled catches were far below the observed ones. The SSB seems to have reached a maximum in 1992, after some years with rapid increase. The fishing mortality appeared to be near zero in that period. After 1992, the results indicate a rapid decline of the SSB and a mortality fluctuating at high levels. The analysis confirms the trends seen in the ordinary assessment, but also confirms the impression that the fluctuation over time have been stronger than the XSA indicates, and there is some indication that the fluctuations in relative survey abundance is stronger than what is compatible with the change in age composition. Thus, this investigation suggests that there may have been variations in the catchabilities in the late 1980’ies or early 1990’ies.
4.3 Management concerns for cod(WD #5 and #6)
It was noted that MBAL = 500 000 t currently used for this purpose doesn’t seem to be well grounded (Borisov et al, WD #5). Strength of year classes to a greater extent depends on the variations in survival conditions of eggs, fry and juveniles (Kiseleva, WD #6). It was proposed to use the revised biological parameters of the population, such as growth rate, maturation, natural mortality coefficients, differentiated by age and years, or to use the reduced time series of observation to reflect the current state of the cod population, as well as to start works on the recruitment modelling taking into account the effect of environmental variations.
4.4 Observations on cod spawning in 2000 (oral presentation by Shevelev)
A considerable reduction in abundance of mature NEA cod on the spawning grounds off the Lofoten showed by the Norwegian acoustic survey in April 2000 can be partly explained by the increased water heat content and the predominance of the first-time spawners in the spawning stock. As a consequence, more cod spawned on the Northern spawning grounds. In early May, Russian research vessels reported about unusually high number (up to 10%) of just spawned cod in trawl catches of the Bear Island.
4.5 Sensitivity to some XSA tuning choices for NEA cod (WD #3)
As described in the 99 WG report here are some peculiar time trends in catchability for most age groups and survey fleets. Some of the important age groups show cyclic patterns with an increasing long term trend. Those trends seem to be significant compared to the year-to-year variability. It has been discussed whether it is caused by changes in the surveys or by variable additional mortality. The 1999 working group did not consider the subject to be sufficiently evaluated to introduce an alternative stock assessment. It was, however, recognised that some changes in survey methodology had taken place, but it is not obvious where each of the actual survey time series should be split. The change may not have taken place in the same year for all vessels involved, and some learning process is involved in most changes, thereby influencing the results for several years.
When there are doubts whether the recent data are comparable to older data, a wise procedure is to reduce the weights
on older data in the tuning. (This is true irrespective of the likely reasons for the discrepancies; surveys, landing
statistics, natural mortality). The time weighting used in the WG tuning is tricubic taper down weighting over 20 years,
which is the program default. To examine the effect of reducing the influence of older data a tricubic taper down
weighting over 10 years was applied. Both when tuning with one survey at a time and when tuning with all fleets in the
standard manner, the pattern was that this down weighting of old data reduced the 99 estimate of population size and increased the estimate of F in 99. Compared to the standard procedure the retrospective pattern improved.
5 REFERENCES
Cook, R.M. 1995. A simple model for the analysis of research vessel data to determine stock trends. ICES CM
1995/D:12.
Table A1 North-East Arctic COD. Catch per unit effort.
Sub-area |I Division IIb Division Iia
Year Norway
2UK
3Russia
4Norway
2UK
3Russia
4Norway
2UK
3Norway
51960 - 0.075 0.42 - 0.105 0.31 - 0.067 3.0
1961 - 0.079 0.38 - 0.129 0.44 - 0.058 3.7
1962 - 0.092 0.59 - 0.133 0.74 - 0.066 4.0
1963 - 0.085 0.60 - 0.098 0.55 - 0.066 3.1
1964 - 0.056 0.37 - 0.092 0.39 - 0.070 4.8
1965 - 0.066 0.39 - 0.109 0.49 - 0.066 2.9
1966 - 0.074 0.42 - 0.078 0.19 - 0.067 4.0
1967 - 0.081 0.53 - 0.106 0.87 - 0.052 3.5
1968 - 0.110 1.09 - 0.173 1.21 - 0.056 5.1
1969 - 0.113 1.00 - 0.135 1.17 - 0.094 5.9
1970 - 0.100 0.80 - 0.100 0.80 - 0.066 6.4
1971 - 0.056 0.43 - 0.071 0.16 - 0.062 10.6
1972 0.90 0.047 0.34 0.59 0.051 0.18 1.08 0.055 11.5
1973 1.05 0.057 0.56 0.43 0.054 0.57 0.71 0.043 6.8
1974 1.75 0.079 0.86 1.94 0.106 0.77 0.19 0.028 3.4
1975 1.82 0.077 0.94 1.67 0.100 0.43 1.36 0.033 3.4
1976 1.69 0.060 0.84 1.20 0.081 0.30 1.69 0.035 3.8
1977 1.54 0.052 0.63 0.91 0.056 0.25 1.16 0.044 5.0
1978 1.37 0.062 0.52 0.56 0.044 0.08 1.12 0.037 7.1
1979 0.85 0.046 0.43 0.62 - 0.06 1.06 0.042 6.4
1980 1.47 - 0.49 0.41 - 0.16 1.27 - 5.0
Spain
6Russia
41981 1.42 - 0.41 (0.96) - 0.07 1.02 0.35 6.2
1982 1.30 - 0.35 - 0.86 0.26 1.01 0.34 6.4
1983 1.58 - 0.31 (1.31) 0.92 0.36 1.05 0.38 7.6
1984 1.40 - 0.45 1.20 0.78 0.35 0.73 0.27 7.0
1985 1.86 - 1.04 1.51 1.37 0.50 0.90 0.39 5.1
1986 1.97 - 1.00 2.39 1.73 0.84 1.36 1.14 4.1
1987 1.77 - 0.97 2.00 1.82 1.05 1.73 0.67 3.3
1988 1.58 - 0.66 1.61 (1.36) 0.54 0.97 0.55 2.2
1989 1.49 - 0.71 0.41 2.70 0.45 0.78 0.43 3.6
1990 1.35 - 0.70 0.39 2.69 0.80 0.38 0.60 4.8
1991 1.38 - 0.67 0.29 4.96 0.76 0.50 0.90
-1992 2.19 - 0.79 3.06 2.47 0.23 0.98 0.65
-1993 2.33 - 0.85 2.98 3.38 1.00 1.74 1.03
-1994 2.50 - 1.01 2.82 1.44 1.14 1.27 0.86
-1995 1.57 - 0.59 2.73 1.65 1.10 1.00 1.01
-1996 0.74 1.11 0.85 0.99
1997 0.61 0.57 0.74
1998 0.37 0.29 0.40
1999
10.29 0.34 0.39
1
Preliminary figures.
2
Norwegian data - t per 1 000 tonnage*hrs fishing.
3
United Kingdom data - t per 100 tonnage*hrs fishing.
4
Russian data - t per hr fishing.
5
Norwegian data - t per gillnet boat week in Lofoten.
6
Spanish data - t per hr fishing.
Period Sub-area I Divisions IIa and IIb
1960–1973 RT RT
1974–1980 PST RT
1981– PST PST
Vessel type:
RT = side trawlers, 800–1000 HP.
PST = stern trawlers, up to 2000 HP.
Table A11 (Continued)
Table A12 North-East Arctic COD. Length at age (cm) from Russian surveys in
Table A13 North-East Arctic COD. Weight (g) at age from Russian surveys in November-December.
Year
Table A14 Abundance indices of 0-group fish in the Barents Sea and adjacent waters in 1965 − 1999.
Table A15 Estimated logarithmic indices with 90% confidence limits of year class abundance for 0-group herring, cod and haddock in the Barents Sea and adjacent waters 1965 − 1999.
Year Herring
1Cod Haddock
85
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