Estimating larval density in cod (Gadus morhua) first feeding tanks using measurements of feed density and larval growth rates

Due to unpredictable mortality in larval cod rearing, a reasonably accurate estimate of the larval biomass in rearing tanks is important for production management such as determination of feed doses and planning of live feed production. A good estimate of larval numbers can also give an early warning if a larval group is suffering high mortality. Because direct measurement of the larval density is difficult, a model based estimator is developed to estimate the larval density from parameters that are simpler to measure, such as feed density and larval size. The estimator is based on an extended Kalman filter using measurements to update a process model.The estimator was tested by acquiring data on feed densities and larval growth rates in a first feeding experiment on cod. Cod larvae were reared from hatching up to day 16 post hatch in nine 160 l tanks at three different densities. The larvae were fed with rotifers (Brachionus plicatilis) from day 3 until the end of the experiment. Results show that the estimator is able to correctly detect differences in larval density.