Estimating the proportion of river and sea spawning whitefish in catches from the brackish Gulf of Bothnia (Baltic Sea) by gill raker counting, genotyping and otolith chemistry analysis

• Main Authors: Henry Hägerstrand, Jan-Olof Lill
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2015-11-01
• Series: Frontiers in Marine Science
• Subjects: genotyping; Baltic Sea; Whitefish; Stocking; otolith chemistry; Gill raker number; otolith elemental concentration
• Online Access: http://journal.frontiersin.org/Journal/10.3389/conf.FMARS.2015.03.00171/full

European whitefish (Coregonus lavaretus) is a commercially and recreationally important species in the Gulf of Bothnia (Figure 1), where the salinity of the surface water increases from ~2 ‰ to ~6 ‰ in the north-south (~725 km) direction. Two sympatric ecotypes with similar outer features occur: stationary sea spawning whitefish and migrating anadromous river spawning whitefish (Lehtonen 1981). The two types mix in sea away from breeding locations and off breeding time. River spawning whitefish can undertake long feeding and spawning migrations, e.g. between the north and the south of the gulf. The river spawning whitefish abundantly occur at feeding grounds in the south where they stay for years, until maturing. Mainly due to anthropogenic destruction of the spawning rivers, the river spawning whitefish has diminished during last decades and is presently listed among endangered species (Helcom 2013). A reliable assessment of the two ecotypes and their subpopulations is a prerequisite for taking effective actions for stock preservation. In order to compare methods for whitefish ecotype identification we undertook genotyping, otolith chemistry analysis and gill raker counting on river and sea spawning whitefish populations. Materials and methods. Whitefish to undergo spawning was sampled from rivers along the Finnish west coast (river spawners) and at sea from known spawning sites close to the southern feeding grounds at the Åland Islands and the Archipelago Sea (sea spawners) (Himberg et al. 2015, Figure 1). Gill rakers were counted on the left outer arch. Data on gill raker number was also collected from literature. Genetic diversity was assessed by nine microsatellite markers (Ozerov et al. 2015). Two methods were used for otolith (sagittae) chemistry analysis. Otoliths were dissolved and analyzed for elemental concentrations with ICP-OES (Hägerstrand et al. 2015), or polished to the core and analyzed for core strontium concentration by PIXE analyses (Lill et al. 2015). Results and discussion. The genetic divergence between river and sea spawning whitefish was low (Ozerov et al. 2015), indicating that genotyping is of little use to differentiate between the two ecotypes on its own. However, genotyping may add to differentiation in combination with other methods such as otolith chemistry analysis and gill raker counting. Otolith chemistry identified naturally reproduced river and sea spawning whitefish based on differences in the strontium concentration in the otolith core region (Lill et al. 2015). Furthermore, bulk otolith elemental concentration analysis (Ba, Sr, Zn, Mn, Mg, Fe) mapped whitefish populations to different regions in the Gulf of Bothnia, and indicated migrating populations (Hägerstrand et al. 2015). Here, barium was shown to be a useful freshwater-seawater (salinity) gradient indicator. Gill raker counting showed that the average gill raker number of river and sea spawning whitefish was significantly different (t=22.50, df=934, p<0.0001), i.e. 29.9±2.14 (n=480) and 26.7±2.21 (n=450), respectively (Himberg et al. 2015). Despite the overlapping distributions of gill rakers between the ecotypes, the average gill raker number can be used to approximate the ratios of the ecotypes and the alterations in them. In conclusion, gill raker counting was verified as an easy, fast and inexpensive method to estimate the spatiotemporal proportion of river and sea spawning whitefish at the southern feeding grounds of the Gulf of Bothnia. The gill raker counting method is well suited to aid in a sustainable management of whitefish stocks. The stability of gill raker numbers in whitefish populations (Himberg et al. 2015) is essential when using this character in approximating the proportion of river and sea spawning whitefish. Interestingly, while the mean number of gill rakers in river spawner populations along the Finnish west coast has stayed stably high (normally ~30) at least during a century (Himberg et al. 2015), the mean gill raker number of sea spawners at the southern Åland Islands (Tengsöda, Geta) has decreased from an average of ~27.5 during 1964-1980 (Himberg 1970, Himberg 1978, Lehtonen 1981, Himberg 1995), to below 26 at today (Table 1, Supplemental Data). This decrease is probably the result of stocking during recent decades of sea spawning whitefish from the local hatchery (Guttorp, Åland Islands), and of the initial selection of brood fish for this activity. The low number of gill rakers of local sea spawning whitefish at the Åland Islands aids in differing it from river spawning migrators. Stocking has substantially increased the abundance of whitefish in the Gulf of Bothnia (Leskelä et al. 2009, Jokikokko and Huhmarniemi 2014). As a result of compensatory stocking to preserve river spawner populations and stocking to increase fisheries catches, a vast amount of the river spawning whitefish “type” fish is released every year. The stocked fish is raised in freshwater ponds. The juvenile fish is released into rivers and river estuaries along the west coast of Gulf of Bothnia, and also into the sea e.g. at the feeding grounds in the southern Archipelago Sea. The fish released into the sea to increase fisheries catches is often of the River Kokemäki whitefish origin since this whitefish is fast growing and gets big (Lehtonen 1981). The proportion in the sea of naturally reproduced and stocked river spawning whitefish is unknown as methods to differentiate between them are not well developed. In the accompanying paper in this series we describe a method that can resolve this issue (Hägerstrand et al. 2015, Front. Mar. Sci. doi: 10.3389/conf.FMARS.2015.03.00113). River spawning whitefish raised in fresh-water ponds and stocked in northern rivers or river estuaries maintain their feeding migration instinct and also return to their home river following maturation (Leskelä et al. 2009, Jokikokko et al. 2014). When stocked in the sea they ascend into rivers close to the site of release (Leskelä et al. 2009). Stocked river spawners can also re-establish populations in rivers from where the river spawning whitefish type has disappeared (Chris Karppinen, personal communication). The homing behavior and spawning characteristics of river spawners stocked in the southern Archipelago Sea far from large rivers are not known. Thus, it remains unclear if these fishes interfere with the spawning of local whitefish and alter their genetic properties.