Wolf fish

Some of the main characteristics are

1.

Simulation period from 1978 to 1999. Four timesteps were used each year

2.

The ages used is 1 to 20 years. The oldest age was treated as a plus group.

3.

3 different alternatives were tested. Natural mortality was varied between alternatives. Natural mortality of age 1 to 17 was 0.15 in alternative 1, 0.1 in alternative 2 and 0.07 in alternative 3. Ages 18 to 20 had higher natural mortality. 4. $L_{\inf}$ not estimated but set to 180 cm to approximate linear growth. Mean length at age from survey indicates that linear growth is a good approximation for this species.

Of the alternatives tested alternative 1 where M=0.15 gave the best fit to data but alternative 3 the worst. The difference was though not great and growth curves and selection pattern were similar for all the cases.

Figures 4.1 shows the estimated recruitment for alternatives 1 and 2. As expected higher values are seen for alternative 1 where M is higher. Recruitment estimates for the youngest age group neet to be investigated further. Allowing non linear relationship between survey indices and number in stock might be nessecary.

  

Figure 4.1: Estimated recruitment at age 1 accoring to the model (altenative 2)

Figure 4.2 shows the estimated growth using M=0.1 and figure 4.3 the estimated selection patterns for the same alternative. In figure 4.2 one standard deviation from the mean is also shown. The figures indicate that at age 10 50% of the fishes have recruited to the fishery.

  

Figure 4.2: Estimated growth of wolffish using M = 0.1

  

Figure 4.3: Estimated selection curves using M = 0.1

Figure 4.4 shows yield per recruit for the 3 alternatives. Alternative 3 (M = 0.07) has the highest yield per recruit and lowest values of F_0.1 and F_max. The values of F_0.1 are 0.23, 0.2 and 0.184 and the values of F_max are 0.43, 0.32 and 0.28.

  

Figure 4.4: Estimated yield per recruit according to model

Figures 4.5, 4.6 and 4.6 show the developent of the catch, catchable, catchable biomass and fishing mortality of age 15 wolffish using F_0.1 after 1999. The recruitment after 1998 is the mean of last 6 years. Age 15 is used to get fishes that are fully recruited to the catch.

  

Figure 4.5: Catch of wolffish. F_0.1 used after 1998

  

Figure 4.6: Catchable biomass of wolffish. F_0.1 used after 1998

  

Figure 4.7: Fishing mortality of age 15 wolffish. F_0.1 used after 1998

The figures indicate that using F_0.1 will lead to increased catch next year if M=0.15 is assumed but unchanged catch if M=0.1 is assumed.

Figures 4.8, 4.9 and 4.10 show development of catch, catchable stock and fishing mortality, of age 15 wolffish using M = 0.1 and 5 different values of F after 1998. It appears from these figures that F = 0.25 will not affect the stock seriously but values of F exceeding 0.25 can not be recommended. F=0.25 leads to a catch of 14,000 tonnes next year which is the same catch as F_0.1 gives according to alternative 1 (M=0.15).

A reasonable catch control seems to be to use F_0.1 and base the assessment on alternative 1 (M = 0.15). This leads to catches which are above F_0.1 for alternative 2 but still lead to increase of catchable biomass.

  

Figure 4.8: Catch of wolffish using M=0.1

  

Figure 4.9: Catchable biomass of wolffish using M=0.1

  

Figure 4.10: Fishing mortality of age 15 wolffish using M=0.1