Genetic analyses reveal complex dynamics within a marine fish management area

Abstract

Genetic data have great potential for improving fisheries management by identifying the fundamental management units—that is, the biological populations—and their mixing. However, so far, the number of practical cases of marine fisheries management using genetics has been limited. Here, we used Atlantic cod in the Baltic Sea to demonstrate the applicability of genetics to a complex management scenario involving mixing of two genetically divergent populations. Specifically, we addressed several assumptions used in the current assessment of the two populations. Through analysis of 483 single nucleotide polymorphisms (SNPs) distributed across the Atlantic cod genome, we confirmed that a model of mechanical mixing, rather than hybridization and introgression, best explained the pattern of genetic differentiation. Thus, the fishery is best monitored as a mixed‐stock fishery. Next, we developed a targeted panel of 39 SNPs with high statistical power for identifying population of origin and analyzed more than 2,000 tissue samples collected between 2011 and 2015 as well as 260 otoliths collected in 2003/2004. These data provided high spatial resolution and allowed us to investigate geographical trends in mixing, to compare patterns for different life stages and to investigate temporal trends in mixing. We found similar geographical trends for the two time points represented by tissue and otolith samples and that a recently implemented geographical management separation of the two populations provided a relatively close match to their distributions. In contrast to the current assumption, we found that patterns of mixing differed between juveniles and adults, a signal likely linked to the different reproductive dynamics of the two populations. Collectively, our data confirm that genetics is an operational tool for complex fisheries management applications. We recommend focussing on developing population assessment models and fisheries management frameworks to capitalize fully on the additional information offered by genetically assisted fisheries monitoring.

Genetic analysis provides insights into species distribution and population structure in East Atlantic horse mackerel (Trachurus trachurus and T. capensis)

“…The results here suggest both species are likely being indiscriminately harvested within this region, which may severely compromise stock sustainability (Healey et al ., ,b; McKeown et al ., ). More powerful genomic‐based assays will also be useful to permit individual‐based analyses of hybridization and fish traceability (Helyar et al ., ; Hemmer‐Hansen et al ., ). Such genomic assays will also be necessary for both species to align spatial management units with patterns of stock recruitment.…”

Assessing the performance of statistical classifiers to discriminate fish stocks using Fourier analysis of otolith shape

“…Here, Baltic cod is managed as two separate stocks: one western stock (ICES subdivisions (SDs) 22-24) and one eastern stock (SDs 24-32; ICES 2019a). The genetically distinct cod stocks coexist in the Arkona Basin (SD 24;Hemmer-Hansen et al 2019;Weist et al 2019), resulting in uncertainties in the stock assessment. Since the ICES benchmark in 2015, otoliths of cod from commercial samples from the mixing area are assigned to their respective stock of origin using elliptic Fourier descriptors and LDA (ICES 2015(ICES , 2019bHüssy et al 2016).…”

Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea

“…In this study, samples were collected from the main distribution area of the Eastern Baltic cod stock. The Western Baltic cod stock mainly inhabits the sea west of the sampling area, but there is also considerable mixing and overlap between the two cod stocks, especially in Arkona Bay (subdivision 24) region (Hemmer‐Hansen et al ., ; Hüssy et al ., ; Weist et al ., ), from which the German samples originated. Therefore, it is likely that a subset of cod included in this study belonged to the Western stock and that the proportion of the two stocks present in samples varied regionally.…”

“…Each country trawled for cod within their national waters in the Arkona Basin, Bornholm Basin, Gdańsk Bay and Hanö Bay regions of the Baltic Sea, respectively, covering the current main distribution of the eastern Baltic cod stock (Hüssy et al ., ; Orio et al ., ) within International Council for the Exploration of the Sea (ICES) subdivisions 24–26 (Figure ). Two genetically distinct but morphometrically similar populations of cod inhabit the Baltic Sea (referred to as the Western and Eastern stocks) and their ranges overlap with considerable stock mixing occurring predominantly in subdivision 24 (Hemmer‐Hansen et al ., ; Hüssy et al ., ; Weist et al ., ). It is therefore likely that samples consisted of a mixture of Western and Eastern Baltic cod.…”