Trawl avoidance as a source of error in estimates of the prevalence of Icthyophonus hoferi disease in Norwegian spring spawning herring (Clupea harengus L.) in the feeding area

Summary: Results are presented from a survey in the Norwegian Sea in the summer 1993, where the abundance and distribution of herring was investigated, using a high resolution sonar and pelagic trawling at the surface. An index of avoidance was constructed by comparing the catch to be expected from the sonar registrations of the number of schools and the amount of scattered registrations, with the actual catch in each haul. This index was negatively correlated to the frequency of diseased herring in the catch. This indicates that diseased herring is less capable of avoiding the trawl. Application of this result for estimating the true disease prevalence is discussed

En verktøykasse for aldersstrukturerte bestandsberegninger basert på fangst- og overvåkingsdata (TASACS)

TASACS (A Toolbox for Age-structured Stock Assessment using Catch and Survey data) is a collection of computer programs for performing routine analytic assessments of fish stocks. It consists of programs for making assessments and a working environment for handling input data, organizing the work and displaying results and diagnostics. The paper gives a detailed description of the methods and an outline of the working environment

Trawl avoidance as a source of error in estimates of the prevalence of Icthyophonus hoferi disease in Norwegian spring spawning herring (Clupea harengus L.) in the feeding area

Summary: Results are presented from a survey in the Norwegian Sea in the summer 1993, where the abundance and distribution of herring was investigated, using a high resolution sonar and pelagic trawling at the surface. An index of avoidance was constructed by comparing the catch to be expected from the sonar registrations of the number of schools and the amount of scattered registrations, with the actual catch in each haul. This index was negatively correlated to the frequency of diseased herring in the catch. This indicates that diseased herring is less capable of avoiding the trawl. Application of this result for estimating the true disease prevalence is discussed

Environmental information for stock evaluation and management advice purposes

This report summarizes the work of an internal working group appointed by the Institute of Marine Research's management group to evaluate inclusion of environmental parameters in stock evaluation. The report discusses the current and potential usage of environmental information and presents specific recommendation on how to increase the usage of environmental information for stock evaluation and management advice purposes. NORSK SAMMENDRAG: Rapporten sammenfatter arbeidet til et utvalg nedsatt av ledergruppen ved Havforskningsinstituttet for å evaluere bruk av miljøinformasjon i bestandsvurdering og rådgivning. Rapporten diskuterer dagens- og potensiell bruk av miljøinformasjon og gir konkrete anbefalinger for hvordan Havforskningsinstituttet kan øke bruken av miljøinformasjon i bestandsvurderinger og rådgivning

Growth patterns in the North Sea and western mackerel in Norwegian catches 1960- 85

Length, weight and age in random samples from Norwegian catches of' mackerel from 1960 to 1985 have been reviewed. A standard growth curve was constructed by applying a modified von Bertalanffy equation to all data. The deviation from this curve is largely independent of age for adult mackerel, and may be used as a measure of the growth pattern. In the North Sea area, the length at age has increased gradually by 4-5 cm during this period, while it has been nearly stationary in the western spawning area. The condition has remained stable in this period. The mackerel caught in the North Sea is generally larger at age than the typical Western mackerel, even in the years and seasons when Western mackerel dominated in the cathes in the North Sea. The mixing of two stock components with different growth patterns also makes the use of length distributions as a measure of age distributions questionable

Stock prediction using stochastic recruitment numbers with empirical stock-dependent distribution

Probability distributions for the recruitment, conditional on the spawning stock biomass (SSB) were made using a kernel method. Predictions were made with recruitments according to these distributions for 10 years, using a Monte Carlo procedure, assuming constant weight at age, maturity ogive and natural mortality. Examples are given for 3 stocks, Western mackerel, North Sea sandeel and North Sea herring. Three management strategies were studied, a fixed F, a fixed yearly catch and a regime aiming at stabilizing the SSB. Due to the variable recruitment, attempts to stabilize the SSB increases the year to year variations in the yield. Attempting to take a fixed catch every year is hazardous, since the appearance of a few poor year classes may detoriate the stock so that the future recruitment suffers. This simple approach may be a useful tool for evaluating management strategies in terms of risks and possible outcome of the fisheries

Ichthyophonus hoferi disease in the herring in Norwegian waters

After the discovery of I. hoferi disease in Norwegian spring spawning herring in summer 1991, a program for systematic surveillance of the disease in the herring in Norwegian waters was developed. The programme, which aims primarily at monitoring the prevalence of the disease, is described. Macroscopic lesions in the heart emerged as the standard diagnostic criterium. The results so far indicate that the situation for Norwegian spring spawning herring is not alarming, altohough it still deserves attention. For the North Sea herring, the situation is less clear. The diseased herring seem to be less prone to follow normal scooling behavior, which causes sampling problems that must be taken into account when interpreting prevalence results

Migration of western mackerel to the North Sea 1973 - 1988

The latest years, it is evident that most of the mackerel present in the North Sea belongs to the Western stock, since the yearly catches by far exceed the presumed size of the North Sea stock. The Norwegian tagging data have been revised to obtain estimates of the percentage of Western mackerel in the catches in the North Sea. By combining these data with the distribution of catches, a reasonable estimate of the migration of the Western mackerel to the North Sea may be obtained

Management with sparse data

We consider a management that aims at controlling the removal from the stock, guided by a perception of the state of the stock derived from limited data that does not allow an ordinary analytic assessment. In simulation modeling, there is a 'real' stock and 'real' removals, and the stock develops according to these removals. The management part creates a feed back loop through a noisy link between the state of the true stock and the actual removals. The performance of the management will depend on how the real removal responds to the stock, and how the stock responds to the removals. Structurally, this is a quite simple feed-back system. The interior of the building blocks is complex and diverse, however, and include obstacles like time lags and stochastic terms. When an analytical assessment is not available, the link between the real stock and the management decisions is harder to understand and model, and it may be noisier. The main emphasis in this paper is finding decision rules that rely on sparse and noisy data. A simulation tool runs as a bootstrap and was made to cover a variety of stocks, decision rules and noise in a versatile way, but on a quite generic level. The link between state of the stock and the basis of the decision was modeled as SSB (or alternatively total stock biomass TSB) derived from the real stock numbers at age, but with random noise and a random year factor. Several types of harvest rules were explored, and pros and cons of various types are highlighted