The objective of the present project is to produce valid estimates of food consumption rates of important predatory fish in the North Sea and the South European Atlantic as input to multispecies modelling, which is a prerequisite for multispecies assessments being a reliable tool in fisheries regulation. The objective will be achieved through the below mentioned research goals by
application of the proposed methodology. The focus is on saithe, haddock, mackerel, horse mackerel, and grey gumard in the North Sea (appropriate data are available on whiting and cod),
and on hake and horse mackerel in the South European Atlantic.
Two different approaches for estimation of food ration are applied in this project. The gastric evacuation method combines field data on the amount of stomach content in a predator with experimental data on the relationship between gastric evacuation rate (stomach emptying rate) and amount of stomach content. The food conversion method combines field data on the growth of a predator with experimental data on food conversion efficiency (the relationship between ration and growth).
Where feasible (i.e. all species except for grey gurnard) both methods are applied for comparative reasons as the field data contributions are subject to unaccountable bias.
Existing stomach data bases have been analysed for composition of prey in categories relevant for our studies as well as for geographical and seasonal coverage, and checked for whether the stomachs have been pooled and analysed in predator length groups or worked up on individual basis.
The work on existing stomach data has enabled us to design the gastric evacuation and food conversion experiments concerning type of prey items versus predator size, and to plan and perform additional sampling of stomachs.
Gastric evacuation experiments have been conducted on saithe, haddock, and horse mackerel and the effects of more of the independent variables estimated in the model proposed by Temming & Andersen (1994).
In the work on establishment of a universal gastric evacuation model for gadoids the effect of temperature seems to be predator specific. The parameter for the shape of the gastric evacuation curve seems to be quite consistent across predator species with a square root model being the candidate whenever different meal sizes enter the evacuation data (Andersen, in press). Further, there are strong indications of the feasibility of grouping fish preys into a few categories regarding prey specific characters.
A simulation program has been developed, which allows investigations on the precision of the gastric evacuation method. The program can handle any evacuation and consumption model, work with groups of fish with different feeding behaviour and at least two different prey species. This program was used to estimate the amount of bias that occurs, if the consumption of a particular prey species is estimated from stomach content data with multiple prey. The deviations between simulated reality and estimates were always moderate and did never exceed 14%.
In the Atlantic a feasibility study concerning which of the predator species hake and megrim to be incorporated into the laboratory part of the project has been carried out. The two species seem to be equally difficult to catch and keep alive and because hake from analyses of existing stomach data showed to be a far more important predator on commercial fish species it is chosen as the species on which to focus for the remaining part of the project time. Experimental work has been abandoned and gastric evacuation rates will be achieved through field experiments as well as by extrapolating results from other gadoids including some experiments on saithe fed blue whiting.
Food conversion experiments are in good progress for saithe, haddock, mackerel, and horse mackerel, where the effects of food ration, body size, temperature and prey type are tested. Measurements of standard metabolism as a function of body size have been performed on horse mackerel.
The field measurements of swimming speed of saithe went out successfully giving some very interesting results on diurnal swimming activity and relationship between body size and average swimming speed.
Andersen, N. G. (in press). The effect of meal size on gastric evacuation in whiting. Journal of Fish Biology.
Temming, A. and Andersen, N. G. (1994). Modelling gastric evacuation without meal size as a variable. A model applicable for the estimation of daily ration of cod (Gadus morhua L.) in the field. ICES Journal of Marine Science 51, 429-438.
The present project aims at
- producing data for estimation of the shape as well as other parameters of the proposed gastric evacuation model (Temming & Andersen, 1994) for natural food items eaten by mackerel, horse mackerel, and grey gumard. Performing some preliminary work on rays.
- contributing data on saithe, haddock, and hake/megrim for parameter fitting of the proposed gastric evacuation model by selecting appropriate combinations of the variables: predator size, food type, food particle size, and meal size.
- and if feasible establishing a universal model for gastric evacuation in gadoids including also cod and whiting as a first step of a modelling work aiming at generalising across predators and prey types.
- contributing additional field data on the amount of stomach content in individual stomachs. Stomach data are in general only on pooled samples by predator size group. At least a representative picture of the weight-frequency distribution of the contents of individual stomachs is needed for an unbiased estimate of ration.
- performing simulation studies to investigate the precision of ration estimates being influenced by the variability of stomach content in the field and gastric evacuation rate data with emphasis on the effect of the choice of a particular gastric evacuation model.
- estimating rations from a growth model based on food conversion experiments with natural food items performed on saithe, hake/megrim, haddock, mackerel and horse mackerel and in addition with activity level included as a variable. For grey gurnard and rays the approach seems inappropriate as there are some problems connected with ageing of these species.
Funding SchemeCSC - Cost-sharing contracts
AB11 9DB Aberdeen