3 December 2003
NFFO-DEFRA MEETING ON FISHERY SCIENCE 12TH NOVEMBER
We agreed that it would be useful to reflect on the points raised in the above meeting, and to put in writing CEFAS' views on Dr Kristjansson's arguments.
Doug Beveridge has supplied us with the Kilkeel power-point presentation, and the Irish Sea paper by Dr Kristjansson. We also have his paper on the North Sea. From these we have identified the specific arguments made, and addressed them individually.
This letter summaries the key questions and CEFAS' responses. The details are given in the attached appendices. DARD have already commented on the Irish Sea presentation (Appendix F), and so we have not revisited those issues specific to the Irish Sea, but we have addressed the more generic points. The summaries are given below.
Dr Kristjansson argues that fishing pressure from the trawl fleet has declined by a factor of 4.5. Noting that ICES has recently said that fishing mortality rates (F) have remained high since the 1980s, he argues that this cannot be due to increased efficiency. His opinion is that the apparent increase in mortality is due to increased natural mortality.
CEFAS (Appendix A): Scottish fishing effort is the more important component of UK effort in the North Sea, and the Scottish data are key to understanding the deployment of UK effort in the North Sea demersal fisheries. Because of changed practices and non-mandatory log-book fields, recent reported Scottish data of hours fishing do not reflect effort deployed, as cited by ICES. The UK data, of kW-days on the grounds, shows recent falls in 2001 and 2002, but this has brought the fishing power of the UK down to the level of the late 1980s or early 1990s. Further decreases are expected in 2003. However, the English demersal effort has significantly declined since the mid 1980s. In contrast, beam trawl effort has increased substantially.
The UK effort series, to 2002, is thus reasonably in accord with ICES determination of a relatively stable F, or a slight decline, over the past decade.
However, ICES should make more effort to relate its estimates of F to trends in international effort, providing an additional means of quality control, and should explain variances between the two metrics (F and effort) where they occur.
I hope this section, with Appendix A, also covers the question raised in your letter of 28 November.
Dr Kristjansson claims that short-term changes in the time series of spawning stock biomass and recruitment appear to be negatively correlated. The negative relationship forms the basis for an hypothesis that density dependent effects are limiting recruitment to the majority of gadoid stocks.
CEFAS (Appendix B): The analytical approach used to demonstrate that spawning stock biomass and recruitment oscillate in an anti-phase is fundamentally flawed. The observation, through time, that when spawning stock biomass is increasing, the recruitment is decreasing and vice versa is an artefact of the filtering algorithm used.
However, recruitment oscillations do sometimes occur in recruitment time series but these generally only occur for brief periods of time. Spawning stock biomass will follow the oscillations after maturity is achieved several years later and this can give the false impression of opposite phase cycles.
In addition, the relative role of the environment and stock size in determining recruitment is not clear but the sensitivity to environmental signals does seem to alter with stock size itself for gadoid stocks such as North Sea cod and Irish Sea cod.
Dr Kristjansson states that fisheries scientists use a 'likely' value for natural mortality (M) that is commonly assumed to be 0.2 for all ages which have recruited into the fishery and argues that M must be variable. Furthermore, the inability to recognise the importance of natural mortality is the worst pitfall for modern fishery management.
CEFAS (Appendix C): Stock assessments depend upon the accurate estimation of natural mortality. Systematic studies have investigated the association between natural mortality and certain life-history traits and by and large, these studies have produced similar natural mortality estimates; e.g. M ~ 0.2 for North Sea cod and M ~ 0.1 for North Sea plaice.
Extensive studies of predator diet in the Arctic, Baltic and North Sea have been incorporated into multi-species stock assessments to estimate natural mortality due to predation. In the North Sea, the inclusion of time-varying natural mortality in the stock assessments has not changed the perception of the historical trends in spawning stock biomass and fishing mortality for those stocks investigated.
For the examples considered, the current biological reference points adopted by ICES still appear appropriate and current levels of fishing mortality are still too high, even if the natural mortality rate is hypothetically doubled.
Dr Kristjansson argues that thinning out of the population of small fish promotes the growth of the remaining fish.
CEFAS (Appendix D): This challenges one of the accepted theories used to manage fish stocks. One of his main points is that current fishing rates should be increased to reduce population size since growth will increase at low population size resulting in higher productivity. Although, weights-at-age decline at high densities, and increase at low densities, density dependence is not strong since a 100% increase in density only results in about a 10% decrease in weight-at-age. The effect of density dependence has been evaluated for North Sea haddock using a population model for a range of constant fishing mortalities over time. Only if fishing mortality is reduced to a level that would support the maximum yield (at an F of about 0.3) is the effect of density dependence noticeable.
Since introducing effort management at Faroe in 1996, fisheries managers have not followed ICES' advice, which has recommended cuts in effort even when stocks are above Bpa. It was claimed that the Faroe effort system is successful because it allows fishermen to follow the natural fluctuations in stocks, and because landings of demersal fish have since increased.
CEFAS (Appendix E):The key issue is that effort levels and TACs have been set above the ICES advice, but the stocks and fishermen are doing well, so ICES must be wrong.
ICES' implementation of the precautionary approach requires that in addition to SSB being above Bpa, fishing mortality should also be below Fpa. ICES continues to recommend cuts in effort at Faroe because the fishing mortality on cod, haddock and saithe is still above their Fpa values.
Even worse, the management plans imply fishing mortalities near to the Flim values.
Stocks will fluctuate naturally, and those at Faroes can be seen to be responding to annual changes in productivity. But, in the long-term, stocks will be affected by recruitments determined by the parent stock size, which can be managed through keeping with the precautionary framework. In the long-term recruitments and the fishery will be put at risk by excessive fishing. This however, does not mean that for transitory periods the stock and the industry cannot be doing well.
Effort management at Faroe may appear to be successful to fishermen because of recent improved catches, but since 1996, when it was introduced, the average fishing mortality on cod and haddock has actually increased. It has achieved the Faroese target F of 0.45 only twice for cod, and only four times for haddock, although for saithe the target has been achieved in most years.
The RCEP has expressed an interest in this review, and I have agreed to send them a copy, and I would be pleased if you could forward a copy to Dr Kristjansson.
Joe Horwood, Defra Chief Fisheries Science Adviser and Deputy to the Chief Executive
Appendix A. (35 kb Word) Changes of fishing effort & fishing pressure measured as fishing mortality.
Appendix B. (213 Kb Word) Note on the article entitled: Fish stocks hard to keep them healthy by Jon Kristjansson, Fishing News, 11 April 2003.
Appendix C. (63 Kb Word). Natural mortality.
Appendix D. (194 Kb Word). Variation in weights-at-age of North Sea haddock and consequences for scientific advice.
Appendix E. ( 120 Kb Word). Management of the demersal fisheries at Faroe.
Appendix F. ( 109 Kb Word). Response by DARD to the report by Jon Kristjansson entitled: On the management of Cod and Haddock in the Irish Sea. A report produced for the Anglo-North Irish Fish Producers Organisation Limited (Dated: August 2003).