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Some biological parameters of the Faroe Plateau Cod
by: Jón Kristjánsson Reykjavík Iceland Introduction
Catch statistics for cod in the Faroe area is available since 1904.
The catch figures are characterised by oscillations which are
exceptionally regular (fig.1). This is interesting and some
attempts have been done to explain this. There are mainly two separate cod stocks inside the Faroe EEZ zone, the (small) Faroe bank stock and the (large) Faroe plateau stock. Vital biological parameters have been recorded since 1961 and in recent years landings from the two stocks have been separated. As the fishery in Faroes is controlled
by effort the catch is supposed to reflect stock size. The
stock size and has been calculated with conventional methods, using fixed natural mortality M of 0.2 and VPA - analysis and survey data to estimate the total mortality Z in order to estimate the fishing mortality F.
It is the opinion of the author that M is highly variable and survey/ VPA- data very inaccurate, leading to inaccurate and unreliable estimates of the calculated stock parameters.
However, this is the best available information, and the only one that can be used to expose relative fluctuations, inaccurate as it is.
Fishery and management
The fishery has bee managed by effort control since 1996. 1994-1996 there was a quota system.
Before that fishery was managed by technical measures. Before the 200 mile EZZ limit was set the
grounds were heavily fished by British trawlers, mainly Scottish. Up to 1995, they fished up to 3 miles
from the shore. That year new ground points were set that closed fjords and bays, and in 1956 a 6 mile
limit was set. In 1964 the limit was increased to 12 miles and in 1974 some trawl free zones were set
outside the 12 mile limit. 200 miles came in 1978 and some areas protected from fishing were introduced.
Mesh size in trawl was 80 mm until 1967 when it increased to 100 mm. 110 mm were allowed in 1970,
135 mm 1978, 155 1990 but put down to 145 mm again the same year.
Materials and Methods.
Data from the ICES working reports on the Faroe plateau cod have been prepared in order to expose some
biological characters in the stock (reveal short term fluctuations in the stock).
In some instances, running three years average was used to smooth short time fluctuations. Nine or thirteen
year running average were used to show long time oscillations. If long term average is subtracted from
the short time average, oscillations around the mean are exposed.
The long time average can also be drawn by hand, before the days of computers, this was usual method.
Data on weight at age are not based on true measured weights, they have been computed from length,
therefore, changes in condition factor is nor reflected. Annual growth rate G of year class i in year n was
calculated from the formula: (Gin= ln (weight i n+1 /weight i n)
Stock size
Stock size is calculated from the landings according to estimated parameters (F and M). Figures are available for the period 1961-2005. Fig. 2 shows landings, fishable stock and spawning Fig. 3, below, shows the fraction that the fishery removes from the stock (calculated).
Fig. 2. Relationship between total biomass, spawning biomass and landings 1961-2005. It is a political decision in Faroes that the fishing pressure should be of such magnitude that it would remove approximately one third of the fishable stock Fig. 3 shows the annual removal (catch rate) from the fishable stock according to VPA-analysis. Most of the time the catch rate is below the political Fig. 3. The fraction that the fishery removed from the total stock 1961-2005.
Weight at age
Mean weight at age of 2-6 year old cod 1961-2005 is
shown in fig 4.
The average weight 3+ and older has been falling through the period which indicates gradual deteriorating feeding condition trough out the period. This may be related to more restrictions put on the fishery, i.e. extended fishery fishery limits to drive the foreign fleet However, the weight of 2+ has increased in the period. That indicates selective fishing. Measures have been taken to reduce fishing pressure (protect small fish) on 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 1 small fish by increasing mesh size in the cod end of Fig. 4. Weight at age 1961-2005 (calculated trawls, thereby the biggest fish in the year class will be over represented - leading to an increase in the mean weight.
Growth rate
To remove the accumulated growth of the various year classes, the annual growth rate G can be The results for 3 and 4 year old fish is shown in fig. It can be seen that growth is fluctuating regularly.
There is a good correlation between the growth of both age classes, they are in phase, showing how growth reflects good and bad environmental Fig. 5. Annual growth rate for 3+ and 4+ cod 1961-2005. Recruitment
Smoothed curve on recruitment is shown in fig. 6, revealing regular fluctuations of 7-9 years.
Fig. 6. Recruitment of cod 1961-2005, smoothed (3 ave).
Growth and recruitment
(smoothed) 1961-2005, and they oscillate in phase throughout the whole period. Growth reflects the general feeding conditions, thus the sensible conclusion is that good feeding condition lead to good recruitment and vice Fig. 7. Growth and recruitment 1961-2005.
Stock recruitment relationship
relationship for Faroe plateau cod 1961-2004. The curves are in opposite phase. When the
recruitment earlier) the recruitment decreases There is an inverse relationship between the 10,000
size of the spawning stock and the
61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99 1 recruitment.
Fig. 8. Spawning stock, SSB (smoothed) , red line, and recruitment, R (smoothed), blue line, 1961-2005).
Total biomass and recruitment
Fig. 9 shows the relationship between total biomass (calculated) and recruitment). Much the same pattern. that is when the stock is big there When the stock is big, there is no "room" for Fig. 9. Stock biomass (blue line) versus recruitment (red line) 1961-2005.
Stock biomass and growth
Fig. 10 shows the relationship between total biomass and growth rate. Growth seems to be inversely related to stock size, i.e. density dependant.
Fig. 10. Stock biomass (red line) and growth (blue line) 1961-2005.
Fishing mortality and growth
Fig. 11 is the relationship between the calculated average fishing mortality F and growth, G.
They are in the opposite phase and the question arises weather "fishing mortality" is in reality natural Fig. 11. Relationship between fishing mortality and growth 1961-2005.
Total mortality, Z - Fishing mortality, F
Average Z
arithmetic mean of the calculated
for the age classes, without
taking into consideration that the year 0.6classes differ in size. old fish and it is most often highest in 0.0the oldest age groups.
Fig. 12. Total mortality, Z, of the 3-7 year old cod 1961-2005.
increases and is always lowest in the youngest fish.) The period after 1987 is interesting and is shown separately Total mortality Z related to growth conditions
This is a plot of the total mortality of the 3-6 year classes 1987 - 2005. Growth rate 4-5 years (smoothed) Periods of high mortality are when the growth is
slow and vice versa, in periods of good growth the
mortality is low. This suggests that the main part of
the mortality is from natural causes, i.e. hunger. In
good growing conditions, mortality is low for all year classes; because M is low and F is is similar for all ages The older fish need more food as they are bigger, therefore they have higher mortality when food is This is the most interesting part of it all: Natural
mortality is not constant for all ages, on the contrary.

Fig. 13. Total mortality, Z, in the 3-6 years old cod 1987-2005. Growth from age 4-5 is superimposed.


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