The world’s oceans are continuously experiencing stronger and stronger fishing pressure, and presently there are few fisheries that are exploited less than to the maximum sustainable yield. In addition, about 30 % of the world’s marine fish stocks are currently fished at biologically unsustainable levels (FAO 2020), a fraction which is currently increasing. As a consequence, a number of actions have been undertaken in order to minimize unsustainable practices, including implementation of no-take zones, fishing quotas, size limits, along with stricter enforcement of regulations (FAO 2020). Many fisheries have also introduced restocking efforts, where fish or shellfish are raised in aquaculture environments from eggs to juvenile life stages, and then released into the wild (reviewed in e.g. Halverson 2008; Taylor et al. 2017). In order to distinguish hatchery-reared individuals from wild ones in later life stages, a variety of different methods have been used, including different types of staining (Camp et al. 2013) and electronic tags (Nzau Matondo et al. 2019). However, the large-scale effects of restocking efforts have to date been difficult to assess, and in many small-scale efforts where evaluations have been possible the costs seem to outweigh the benefits (Kitada 2018).
The European Lobster (Homarus gammarus) is the base for an important local fishery all along the coast of Europe. Despite this, there are currently no EU-common regulations for the fishery (Bryhn et al. 2020). Along the Swedish west coast, the lobster fishery is to a large extent recreational, and although there is no record of the number of landings from the recreational fishery, it was estimated to account for almost 90 % of total Swedish used traps, and 75 % of total landings in 2007 (Bryhn et al. 2020). Lobster stocks have not been officially monitored in Swedish waters to date, making it difficult to assess whether they are overfished, but the number of landings as well as the catch per unit effort have both steadily decreased since the 1960s, indicating that the stock is declining (Sundelöf et al. 2013; Bryhn et al. 2020). Similar trends have also been seen in other North Sea lobster stocks, in Norway and in the UK (Bryhn et al. 2020). As a consequence, regulations for the recreational fishery have been tightened in recent years in Sweden, with a lowered number of traps per person allowed, and higher minimum landing size limits.
In addition to the stricter regulations, a restocking effort has recently been introduced on the Swedish west coast, where ovigerous females are supplied by local lobster fishers, and the larvae are hatched in aquaculture facilities. After that, the lobster larvae are reared until ready to settle on the benthos, after which they are released at selected sites in the same area where their mothers were caught. Restocking has been taking place since 2018, with the expectation that the first outplanted lobsters will have grown above the minimum landing size in the fall of 2022. Estimating the success of the restocking effort will be of crucial importance to the fishery at that point.
In order to accurately determine if a caught lobster is laboratory-hatched or not, genetic analyses are optimal (Ellis et al. 2015b). With known parental genotypes, it is possible to assess whether an individual juvenile is the progeny of any of the known genotypes or not, given a large enough panel of genetic markers. These so-called assignment tests have been used for parentage analysis for a long time, based on microsatellite markers (Ellis et al. 2015a). More recently, however, single nucleotide polymorphisms (SNPs) have replaced microsatellites as the most commonly used markers due to the abundance of such markers in most genome sequences, and the ease and reproducibility of genotyping (Flanagan & Jones 2019). Recently, a novel SNP panel of 96 markers was developed to test for population differentiation and population assignment among European stocks of H. gammarus (Jenkins et al. 2019a). This panel was shown to work well for the intended purpose (Jenkins et al. 2019b). However, it has to date not been tested for its power of parentage assessment.
Here, we applied the SNP panel developed by Jenkins et al. (2019a) to a dataset consisting of mothers, progeny and unrelated control individuals, in order to test the power to: (1) Assign juveniles to the correct mother; (2) Not incorrectly assign unrelated individuals to a mother. As pregnant females collected for the restocking effort in the wild were used, there was no information on potential father(s), so therefore an additional goal was to assess the power of the panel to: (3) Confidently reconstruct paternal genotypes. In addition, the panel was used to (4) Investigate whether clutches were originating from one or several fathers.