Abstract
With the increasing anthropogenic impacts on fish habitats, it has become more important to understand which primary resources sustain fish populations. This resource utilization can differ between fish life stages, and individuals can migrate between habitats in search of resources. Such lifetime information is difficult to obtain due to the large spatial and temporal scales of fish behavior. The otolith organic matrix has the potential to indicate this resource utilization and migration with δ13C values of essential amino acids (EAAs), which are a direct indication of the primary producers. In a proof‐of‐concept study, we selected the Acoupa weakfish, Cynoscion acoupa, as a model fish species with distinct ontogenetic migration patterns. While it inhabits the Brazilian mangrove estuaries during juvenile stages, it moves to the coastal shelf as an adult. Thus, we expected that lifetime resource utilization and migration would be reflected in δ13CEAA patterns and baseline values in C. acoupa otoliths. By analyzing the C. acoupa otolith edges across a size range of 12–119 cm, we found that baseline δ13CEAA values increased with size, which indicated an estuarine to coastal shelf distribution. This trend is highly correlated with inorganic δ13C values. The δ13CEAA patterns showed that estuarine algae rather than mangrove‐derived resources supported the juvenile C. acoupa populations. Around the juvenile size of 40 cm, resource utilization overlapped with those of adults and mean baseline δ13CEAA values increased. This trend was confirmed by comparing otolith core and edges, although with some individuals potentially migrating over longer distances than others. Hence, δ13CEAA patterns and baseline values in otoliths have great potential to reconstruct ontogenetic shifts in resource use and habitats. The insight could aid in predictions on how environmental changes affect fish populations by identifying the controlling factors at the base of the food web.
Highlights
Fish populations are increasingly influenced by human activities such as removal of estuarine vegetation for rural development, discharge of chemicals, climate change, and ocean acidification (Cheung, Brodeur, Okey, & Pauly, 2015; Halpern et al, 2008; Jackson, 2001; Nagelkerken, Russell, Gillanders, & Connell, 2015)
Inorganic δ13C values and mean baseline δ13C patterns of individual EAAs (δ13CEAA) values in the same otoliths of C. acoupa individuals ranging between 12 and 119 cm standard length (SL) showed the same trend of becoming more positive with increasing SL
We investigated the extent in which baseline independent δ13CEAA patterns can differentiate among otoliths of adults and juveniles and the various primary producer groups
Summary
Fish populations are increasingly influenced by human activities such as removal of estuarine vegetation for rural development, discharge of chemicals, climate change, and ocean acidification (Cheung, Brodeur, Okey, & Pauly, 2015; Halpern et al, 2008; Jackson, 2001; Nagelkerken, Russell, Gillanders, & Connell, 2015). The known distribution of the Acoupa weakfish, Cynoscion acoupa, from mangrove estuaries in the early life stages to offshore coastal shelf areas as adults (Barletta, Barletta-Bergan, & Saint- Paul, 1998; Barletta, Barletta-Bergan, Saint-Paul, & Hubold, 2003) suggests an ontogenetic migration between the two habitats and changes in resource utilization. It is a commercially important species occurring along the entire coast of Brazil, little is known about its foraging ecology or migration. We analyzed otolith edges among different size classes and compared individual otolith cores and edges
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