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We here discuss the scope, nature and quality standards of simulation models that may be used in order to evaluate proposed or candidate management plans, including their parameters (reference points). A recently proposed management plan for North-East Arctic cod is used as a working example. We discuss some generic designs of management plans, in particular with respect to performance in relation to different kinds of objectives, including adherence to the precautionary approach. We further discuss simulation tools to evaluate the performance of management plans and to select appropriate values for their key parameters, and suggest some quality criteria for such tools. In particular, we stress the need to have unbiased stochastic parameters and to have realistic levels of errors in the basis for future decisions according to the plan. A new management plan for North-East Arctic cod (Gadus morhua) was agreed by the management agency (Joint Norwegian/Russian Fishery Commission) in 2002. The main elements in this plan are: • Estimate the average TAC-level for the 3 following years based on Fpa. TAC for the following year is set on the basis of this average TAC level. • The following year the estimation of the TAC-level for the next 3 years is repeated based on updated information on stock development. However, the change of TAC from one year to the next cannot be more than +/-10%. This relatively complex plan is used as an example, and aspects that need to be considered in order to evaluate how it can be expected to perform, are discussed.

Working Document #8, Arctic Fisheries Working Group, San Sebastian 23 April- 2 May 2003

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Medium-term simulations on the stock development of Norwegian spring-spawning herring (Clupea harengus L.) have been carried out since 1994 by the ICES «Atlanto-Scandian Herring and Capelin Working Group», in 1996 renamed «Northern Pelagic and Blue Whiting Fisheries Working Group. The results of the simulations have been used as a basis for advice on harvest control rules for Norwegian spring spawning herring. The present paper reviews developments of medium-term simulations carried out on this stock and results from mediumterm forecasts carried out in 1995 and 1999 are used as examples. Emphasis is put on the usefulness of the medium-term simulations in the process towards reaching an agreement on a harvest control rule in 1999.

A high growth rate for Arcto-Norwegian cod (Gadus morhua L.) in the Barents Sea and adjacent areas from the larval stage to the 0-group stage enhances survival and ultimately recruitment to the fishery. However, it appears that relatively high growth rates for a cohort through the 0-group stage are not continued as the cohort ages. Based on survey data, there is a negative, though not significant, correlation between a cohort’s average length at the 0-group stage and its average length as 1 year olds and a significant negative correlation between the average length at the 0-group stage and its average length at ages 2 through 9. It appears that the reason for this slowing of growth is that a strong inflow of warm, prey-rich Atlantic water into the Barents Sea from the Norwegian Sea provides favorable conditions for growth a”nl’d hence survival of the cod larvae and juveniles. But this same strong inflow cakes a proportion of the cohort farther to the east in the Barents Sea, where the bottom water is colder than in the west. Thus the young settled cod experience colder conditions, which slows their growth, than those cohorts that have a more westerly settlement, and this slow growth before age 2 appears to be the reason for the cohort’s relatively smaller mean length at older ages.

The overall objective of the STEREO project is to improve the methodology for determining limit reference points for the biomass of exploited fish stocks. Limit reference points set boundaries which are intended to constrain harvesting within safe biological limits, and are integral components of the decision making process in fisheries management. In most cases limits are currently estimated from historical time series data on spawning biomass and recruitment. However, the underlying stock-recruitment relationship is often poorly defined by such data and hence there is considerable uncertainty around many reference limits. This project will produce an operational scheme for refining functional form of stock-recruitment relationships by incorporating biological, spatial and temporal information on the stock structure, with the aim of reducing the uncertainty associated with the derived limits. The methodology will be developed for cod and haddock stocks around Iceland, Norway and in the North Sea as case studies.

Working Document, ICES Arctic Fisheries Working Group, Copenhagen 23 August – 1 September 1999.

Indices of the abundance of Northeast Arctic cod (Gadus morhua L.) for various life stages were compared in order to determine the earliest stage at .which year-class strength is established. The indices considered are: VPA estimates of spawning stock biomass; an egg abundance index; an early juvenile (approximately three-months old) abundance index; 0-group (age four to five months) abundance indices; bottom trawl and acoustic survey abundance indices for one, two and three- year-old cod and VPA estimates of the abundance of three-year-old cod. Based on a regression analysis, the relative abundance of early juvenile cod was the first life stage that was correlated with their relative'abundance as three year olds. Since the cokelation between the estimates of spawning stock biomass or total egg production and the relative abundance of three-year-old cod was weak, it is concluded that considerable yearly variation in mortality occurs before cod reach the early juvenile stage.

Precision and relevance of pre-recruit studies for fiskery management related to fish stocks in the Barents Sea and adjacent waters. Proceedings of the sixth IMR-PINRO Symposium Bergen, 14-17 June 1994. The predation by North-East Arctic cod on pre-recruits of cod and haddock is reviewed based on data from the joint PINRO-IMR stomach content data base. It is investigated how the abundance indices from surveys of pre-recruits are correlated with the occurence in cod stomachs of pre-recruits of these species. Estimates of the consumption of cod and haddock by cod based on a new model for the stomach evacuation rate of cod are also given.

Working Document to the Atlanto-Scandian Herring, Capelin and Blue Whiting Assessment Working Group, Bergen 12-18 October 1995

Stomach content data collected from cod caught during offshore resource assessment surveys in three Arcto-boreal ecosystems (the Barents Sea and the shelves off Iceland and eastern Newfoundland) were examined to determine the prevalence of cannibalism, and to identify possible factors associated with variability in prevalence. Cannibalism provides a minor source of food, except for the largest predators. Cannibalism Increases with predator length. Most prey are less than 40 cm In length and less than 3 years old. In each region, cannibalism occurs over most of the area surveyed, but is most prevalent where the smallest cod have been found in bottom trawl surveys. The frequency of occurrence of cannibalism recorded in recent years (late 1970s to the present) has usually been low (

Interrelations between fish populations in the Barents Sea. Proceedings of the fifth PINRO-IMR Symposium. Murmansk, 12-16 August 1991. The Barents Sea capelin stock coliapsed during the period 1983-1986. One of the main factors causing the colIapse was a rapid increase in consumption of capelin by cod, caused by the strong 1983 cod year class. Based on measurements of stomach evacuation ra+es in the appropriate temperature interval and data from a combined Russian-Norwegian stomach sampling programrne, predation mortalities are estimated by the MR multispecies model for the Barents Sea - MULTCPEC. In order to estimate these predation mortalities, cod-capelin interaction parameters and yearly migration parameters for capelin are ako estimated.

Interrelations between fish populations in the Barents Sea. Proceedings of the fifth PINRO-IMR Symposium Murmansk, 12-16 August 1991. Diurnal feeding rhythms and daily diets of the North-East Arctic cod were investigated based on data obtained at several stations made in the summer of 1988-1989. A well-pronounced feeding rhythmicity was recorded under conditions of a stable occurence of food organisms and in case cod fed on a single prey species. The daily rations of cod which fed on Euphausiacea, Themisto, fish and shrimps varied from 2.10 to 3.7 % and from 0.28 to 0.99 % of the body weight in 1988 and 1989, respectfully.

Interrelations between fish populations in the Barents Sea. Proceedings of the fifth PINRO-IMR Symposium Murmansk, 12-16 August 1991. The North-East Arctic cod stock's consumption of different prey species has earlier been calculated by Mehl (1989) for the period 1984-1986. However, a new model to describe gastric evacuation on cod has now been presented by dos Santos (1990). This model is based on extensive feeding experiments with relevant prey items, done at relevant tempreature conditions. This model is combined with the stomach content data to give consumption estimates for each of the years 1984-1989. The results are compared and judged against data on the individual cod growth, daily rations and food conversion efficiency.

Working document to the ICES Arctic Fisheries Working Group, Copenhagen 20-28 August 1997

The effect of survey design on the precision of estimates of average fish stomach contents is examined. The contribution to the total variance from within and between tow variability in stomach contents is evaluated, and the implications for stomach sampling programs are discussed. As an example we have estimated the average amount of capelin in Barents Sea cod stomachs for years of very low, low and medium capelin abundance. The results indicate that to maximize precision for a given cost it is generally best to sample stomachs at as many locations as possible. A simulation study based on resampling from these data suggests that little is gained in precision by collecting 5 instead of 2 stomachs from each 5 cm length group of fish.

The Barents Sea capelin stock collapsed during the period 1983-1986. One of the main factors causing the collapse was a rapid increase in consumption of capelin by cod. Based on measurements of stomach evacuation rates in the appropriate temperature interval and data from a combined Soviet-Norwegian stomach sampling programme, predation mortalities are calculated by a simplified version of the IMR multispecies model for the Barents Sea - MULTSPEC. Cod-capelin interaction parameters and migration parameters for capelin in MULTSPEC are also estimated. The use of the predation mortalities calculated from the multispecies model in a single-species model used for capelin management is discussed.

The consumption rares in fish are important but also often weak points in multispecies models. In the Barents Sea multispecies model (MULTSPECI, as in many other models, quantltative stomach content data are combined with gastric evacuation data or models to produce consumption rates of important prey species. To evaluate the evacuation model presently used for cod in the Barents Sea and the derived consumption rates, we have combined stomach data from 1984 with five other evacuation models for cod. The results are compared and judged against data on prey stock sizes, individual cod growth, daily rations and food conversion efficiency. The rations derived from the different models ranged from 0.5 to 1.22 of the BW daily on average for all agegroups of cod or from 2.1 to 4.4 times the biomass of the total cod stock annually. Food conversion efficiency varied between 6 and 591 for the different models and agegroups.

Working document to the ICES Multispecies Working Group meeting, December 1990.