title: Environmental drivers and body condition of coastal fish populations from Brazil and the North Sea
Creators: name: Gomes, Marcelo
Publisher: name: University of Liverpool Repository
Year: 2021

description: Environmental drivers are defined as environmental shifts that cause a quantifiable change in a biological response. The accurate determination of environmental influence on populations is of global importance, and such influence may be observed from the individual to the ecosystem level. Coastal areas include several types of interconnected habitats contained between the nearshore region and the continental slope, and a given coastal fish population is able to make use of many or most of them. The dynamics of fish populations in coastal habitats are regulated by the variety of resources available, which are crucial for population maintenance. Body condition is defined as a measurement of morphometric or physiological body state, and used as an indicator of health status. Environmental changes may also affect metabolism on a seasonal, rapid scale or in the long term, and estimates of body condition are helpful in order to predict group-specific responses to environmental drivers in the population. In this thesis I aimed to determine the influence of environmental drivers on fish individual body condition, directly or via influencing resource availability and fish population structure. A conceptual model on the pathways link these four factors was designed in order to set the objectives of this thesis. Linear regression models fitted with log10 of the weight as the response variable, and log10 of the length plus relevant variables related to environmental drivers and fish biology as explanatory variables were used to this end. Effects of environmental drivers on body condition of individuals were estimated using derivatives of predicted weight with respect to these variables. From these individual effects, the population response to these drivers was calculated. I first determined whether the presence of detached macroalgae would be considered an environmental driver for fish populations in the southeastern coast of Espírito Santo State, Brazil (20,6o - 21o S). Using δ13C and δ15N stable isotope analysis I determined a trophic chain involving fish, invertebrate prey and macroalgae, but the main source of organic carbon to higher trophic levels remains unclear. I found evidence that fish were heavier for a given length in the dry period, suggesting resource availability, other than the expected riverine ones, to those individuals. A statistical model for the effects of environment on the length-weight relationship was developed. Building on such models, I measured the effects of River Itapemirim runoff volume and macroalgae density on body condition of 12 fish groups. I confirmed that detached macroalgae should be considered an environmental driver of body condition for fish populations at the local to regional scales, and its influence is as strong as the acknowledged estuarine dynamics, possibly in a synergistic way. The same approach was applied for determination of warming as an environmental driver for nine commercial fish populations in the North Sea. I investigated the predicted effects of sea surface temperature on body condition of individuals between 1996 and 2017. I tested how the relationship between temperature and condition depended on aspects of distribution, size and age structure through the following hypotheses: the responses of body condition to warming were 1) positive in Lusitanian, pelagic and southwards-shifting species; 2) positive mainly in smaller, younger individuals in northwards-shifting (Boreal or Lusitanian) species and 3) less negative in smaller than larger individuals at temperatures above their established optimal thermal range. Results were consistent with the hypotheses, but varied among the studied species. I examined the relationship between body condition and density for eight out of these nine species. I found evidence of a small negative effect of density on body condition, which could be explained by spatial differences in resource availability. Thus, the results of this thesis corroborated some pathways of the proposed conceptual model, while others have found no support. This thesis provides substantial evidence of the usefulness of our approach for evaluation of environmental influences on fish populations. In the long term, this approach may become suitable for analyses involving phenotypic trends and selection pushed by environmental and/or anthropogenic pressures.