Physiology, phenology and behavioural strategies of forage fish : Studied by bioenergetic modeling

Abstract

Forage fish are small individuals, and are very abundant in numbers and can form dense schools. Forage fish are important within the food webs of the oceans, as they are at the lower trophic levels. Forage fish prey on zooplankton and they are themselves preyed on by piscivore fish. The individual forage fish and its growth dynamics are governed by an interplay between physiological rates, e.g. metabolism and consumption and the ambient environment as the rates are temperature dependent. The topic of this thesis is to describe the strong link between the individual and the environment through bioenergetic modeling. The bioenergetic model is based on the Wisconsin framework, and assumes that ingested energy is deducted losses to egestion, excretion, standard dynamic action and metabolic costs. Surplus energy, if available, is divided between growth in two structure pools; somatic tissue and reserves. The model includes an additional structure pool; gonads, to which energy is transferred during the spawning season. During periods of poor feeding, energy to cover metabolic costs are firstly taken from the reserve pool and secondly, if the reserves are depleted, from the somatic tissue pool. The model is forced by the ambient temperature experienced by the individual and prey. The ioenergetic model reveals how the physiology and the link to the environment are governing factors determining the growth dynamics of the individual. The model includes evaluation of the life history of an individual allowing study of individual behaviour, making the model a strong tool, for investigating hypotheses of life history and behavioural strategies in forage fish. The model is parameterized for Baltic Sea sprat and was used for determination of the importance of temperature on physiology, growth and egg production of adult female sprat. Particular emphasis is placed on the allocation of energy inside the individual, whether energy is invested in growth or reproduction. The simulations reveal that a constant division of surplus energy between the lipid and soma pools leads to seasonal variations in growth and size matching observations. Also an increase in the soma growth potential as a function of temperature was observed, but that the maximum size will decrease with increasing temperature. Temperature is especially important for Baltic sprat as the Baltic Sea is at the species northernmost distribution limit with regard to temperature. Climate scenarios for the Baltic Sea predict a significant warming and the impact of predicted future climate changes and the prey phenology on growth, egg production and fitness was determined. We find the warmer future climate reduces egg production and fitness. The larger the temporal match of the prey with spawning season, the larger is the egg production and fitness. Therefore the effects of temperature on the sprat stock can be lowered if prey phenology responds to the climate. The bioenergetic framework is also used to investigate effects of individual behaviour on the fitness of an individual. Baltic Sea herring are observed to skip spawning when individuals have low condition. This phenomenon and its implications to individual fitness are quantified and are linked to the individual and environmental conditions as well as spawning season. Changes in the individual condition were affecting the number of skipped spawning events and age at first spawning. Spring spawning was found to be the better strategy compared to autumn spawning. Another behavioural strategy which was investigated was the burrowing strategy seen in 5 sandeel in the North Sea. The trade off by this strategy whereby the energy intake is lowered but the strategy also reduces potential predation is quantified. This revealed that the strongest trade off for the individual is to reduce mortality rather than to seek an opportunity to grow larger and thereby be able to produce more eggs