Density-dependence in space and time: opposite synchronous variations in population distribution and body condition in the Baltic Sea sprat (Sprattus sprattus) over three decades.
Spatio-temporal density-dependent processes are crucial regulatory factors for natural populations. However, there is a lack of studies addressing spatial density-dependence in fish growth. A previous investigation has suggested spatio-temporal density-dependence in body condition of Baltic sprat. Here, we used different techniques, such as centre of gravity, distance, and homogeneity indices, to better characterize the spatial and temporal variations in sprat density and body condition in the Baltic Proper. Our results evidenced a negative spatio-temporal co-variation between the centres of gravity of density and maximum condition. In the 1980s-early 1990s both centres were located in the middle of the Baltic Proper. From the mid 1990s the centres progressively separated in space, as the sprat population moved towards the north-eastern Baltic Proper, and the centre of maximum condition towards the south-western areas. Moreover, at low abundances, sprat density and condition were homogeneously distributed in space, whereas at high abundances both density and condition showed pronounced geographical gradients. The ecological processes potentially explaining the observed patterns were discussed in the light of the Ideal Free Distribution theory. We provide evidence that the shift in the spatial distribution of cod, the main predator of sprat, has been the main factor triggering the overall spatial changes in sprat density, and thus condition, during the past thirty years. The spatial indices shown here, synthesizing the spatio-temporal patterns of fish distribution, can support the implementation of the EU Marine Strategy Framework Directive.
Swedish University of Agricultural Sciences, Department of Aquatic Resources, Institute of Marine Research, Lysekil, Sweden.
Michele
M
Casini
University of Oslo, Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, Oslo, Norway; UMR 212 EME, IFREMER (Institut Français de Recherche pour l'Exploitation de la mer), Sète, France.
Tristan
T
Rouyer
Swedish University of Agricultural Sciences, Department of Aquatic Resources, Institute of Marine Research, Lysekil, Sweden; University of Gothenburg, Department of Earth Sciences, Gothenburg, Sweden.
Valerio
V
Bartolino
Swedish University of Agricultural Sciences, Department of Aquatic Resources, Institute of Marine Research, Lysekil, Sweden.
Niklas
N
Larson
National Marine Fisheries Research Institute (NMFRI), Department of Fisheries Resources, Gdynia, Poland.
Włodzimierz
W
Grygiel
D000818:Animals; D001454:Baltic States; D044822:Biodiversity; D001823:Body Composition; D005399:Fishes / Q000033:anatomy & histology* / Q000502:physiology*; D006112:Gravitation; D009792:Oceans and Seas; D011157:Population Dynamics; D013997:Time Factors