Abstract
Unobserved diversity, such as undetected genetic structure or the presence of cryptic species, is of concern for the conservation and management of global biodiversity in the face of threatening anthropogenic processes. For instance, unobserved diversity can lead to overestimation of maximum sustainable yields and therefore to overharvesting of the more vulnerable stock components within unrecognized mixed‐stock fisheries. We used DNA from archival (otolith) samples to reconstruct the temporal (1976–2011) genetic makeup of two mixed‐stock flounder fisheries in the Åland Sea (AS) and the Gulf of Finland (GoF). Both fisheries have hitherto been managed as a single stock of European flounders (Platichthys flesus), but were recently revealed to target two closely related species: the pelagic‐spawning P. flesus and the newly described, demersal‐spawning P. solemdali. While the AS and GoF fisheries were assumed to consist exclusively of P. solemdali, P. flesus dominated the GoF flounder assemblage (87% of total) in 1983, had disappeared (0%) by 1993, and remained in low proportions (10%–11%) thereafter. In the AS, P. solemdali dominated throughout the sampling period (>70%), and P. flesus remained in very low proportions after 1983. The disappearance of P. flesus from the GoF coincides in time with a dramatic (~60%) decline in commercial landings and worsening environmental conditions in P. flesus’ northernmost spawning ground, the Eastern Gotland Basin, in the preceding 4–6 years. These results are compatible with the hypothesis that P. flesus in the GoF is a sink population relying on larval subsidies from southern spawning grounds and the cause of their disappearance is a cessation of larval supply. Our results highlight the importance of uncovering unobserved genetic diversity and studying spatiotemporal changes in the relative contribution of different stock components, as well as the underlying environmental causes, to manage marine resources in the age of rapid anthropogenic change.
Highlights
Unobserved genetic diversity, whether in the form of within‐species genetic structuring or of the presence of cryptic species, is of major importance for the conservation and management of global biodiversity in the age of rapid environmental changes (Bickford et al, 2007; Dirzo et al, 2014)
The use of historical samples allowed us to discover a hidden turnover of floun‐ der assemblages and to demonstrate this turnover coincided tempo‐ rarily both with the decline in flounder stock and with the decline of environmental conditions in the northernmost spawning grounds of P. flesus
The decrease in flounder abundance in the Gulf of Finland (GoF) does not reflect the gradual decline of a single population, but rather the disappearance of P. flesus and the successive decline of P. solemdali, which in turn are determined by the degradation of environmental conditions at local and regional scales
Summary
Unobserved genetic diversity, whether in the form of within‐species genetic structuring or of the presence of cryptic species (morpho‐ logically indistinguishable biological groups incapable of interbreed‐ ing), is of major importance for the conservation and management of global biodiversity in the age of rapid environmental changes (Bickford et al, 2007; Dirzo et al, 2014). The suitable spawning habitat of P. flesus, which has always been geographically limited to higher‐salinity deep‐ water spawning grounds (Nissling et al, 2002, see Figure 1a), has contracted in recent decades, resulting in fluctuations in both stock size (Orio et al, 2017; Ustups et al, 2013) and larval survival (Hinrichsen et al, 2017), in the EGB This could have reduced larval subsi‐ dies and adult spillover to the Gulf of Finland, providing a plausible explanation for the declining trend seen in this area. | 553 with environmental conditions favorable to pelagic spawning in the EGB, (b) there should be a temporal decrease in the proportion of P. flesus in the GoF reflecting worsening environmental conditions in the central Baltic Sea, and (c) this temporal trend should coincide in time with the reported stock decline in GoF
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