National monitoring programme to include genetic diversity in fish stocks

Published on September 17, 2021

A science seminar was held at Marint Centrum (The Marine Centre) in Simrishamn, Sweden, on the 10th of September, focusing on the role of genetic diversity as a pillar within ecosystem-based management. Several recent studies on genetic sub-populations of herring were discussed, as well as the shortcomings of the current fisheries management of the herring stocks in the Baltic Sea.

Leif Andersson, professor in Functional Genomics at Uppsala University and guest professor in Molecular Animal Genetics at Swedish University of Agricultural Sciences, discussed the genetic adaptation of the Atlantic herring to the Baltic Sea. Herring is one of the most abundant fish species on a global scale and historically it has been a cornerstone in the diet of the people living around the Baltic Sea.

Genetically speaking, the herring has adapted to the light conditions of the Baltic Sea. There are distinct genetic differences between herring spawning in the south Baltic Sea, the eyes of which are adapted to more red light, and the herring in the North Sea and along the Norwegian coast, which are adapted to more blue light. In addition, spring spawners and autumn spawners in the Baltic also differ genetically. However, although genetically different, spring and autumn spawners can change their spawning season during certain circumstances, often driven by temperature. Recently, the autumn spawners have increased in the Baltic Sea and spring spawners decreased – why this is the case is not clear.

What is clear though, is the fact that there are genetically distinct sub-populations in Gamleby and Gävlebukten in Sweden, but the number of sub-populations and their exact geographical range is unknown. Further uncertainties include population stability over time, as well as the time frame for recolonization of areas with a sub-population collapse. However, new research and data collection is underway and these issues will be investigated further.

The Swedish Agency for Marine and Water Management (SWAM), together with the Swedish Environmental Protection Agency (SEPA), has the ambition to include genetic diversity in environmental monitoring. A national programme will be initiated and in the pilot phase herring will be one of four aquatic species investigated. The aim is to study how many sub-populations there are in Swedish Baltic coastal waters, where they spawn and whether the fishery is genetically sustainable, i.e. check the fishing pressure on each sub-population. Cod will also be included in the new monitoring programme. The initiative is a collaboration with SLU Aqua, coastal fishermen, authorities and other actors, as well as several research teams from Stockholm University, SLU, Uppsala University and Gothenburg University. Currently sampling is in progress.

Genetic variations in ecosystems are imperative to biodiversity and it strengthens ecosystem functions, as discussed by professor Kerstin Johannesson, Gothenburg University, and professor Linda Laikre, Stockholm University. Large genetic variation equals increased adaptability, high potential for survival over time.  In contrast, low genetic variation is linked to poor adaptability, weak potential for survival and frail resistance. Genetic variation determines whether species will be able to adjust to changes in environmental conditions. Hence, genetic variation is important for biodiversity and ecosystem functions and should be part of the management thereof, especially if applying ecosystem-based management. It’s also a current issue, with the new UN biodiversity strategy highlighting the importance of genetic variation.

The management of herring was discussed by Ulf Bergström, SLU Aqua. The fishing practices have changed over the years from small-scale fisheries to several large trawlers. Catches have decreased over time to about half the amounts landed during the peaks in the 1980s. The Maximum Sustainable Yield (MSY) target is central to European fisheries policy, but there are several uncertainties associated with the stock assessments which forms the basis of MSY application. A key element for the stock assessment is the ‘spawning stock biomass’ (SSB), which – for Central Baltic herring – has been overestimated for years with a miscalculation of almost 100%, i.e. the SSB was considered almost double the size of what was perceived in retrospect. With almost a decade of overfishing, the herring in the central Baltic Sea is in sharp decline and it is clear that uncertainties need to be better addressed in the stock assessments.

Currently, coastal fishermen in Sweden experience a shortage of herring, especially the large individuals which are crucial for the coastal fishery. At the same time, the SSB of the Bothnian Sea is considered to be large. Explanations offered for this disparity include the MSY target not adhering to the size distribution of a stock or local sub-populations, and also increased seal predation. However, the mortality of fish caused by birds and mammals is calculated to be at most  10 % compared to 90 % from fishing.  Increased knowledge is needed in terms of population structure, factors for growth, the shift between spring spawners and autumn spawners and also the role of the herring in the wider ecosystem.

There are several management objectives for fish and fisheries, which are not always properly aligned. The MSY target aims to maximise the commercial catches of fish while remaining ‘sustainable’ in the long term. But there are other objectives as well, for example the qualitative descriptors for determining good environmental status (GES), (3) and (4) in ANNEX I in the Marine Strategy Framework Directive (MSFD), stating the importance that “(3) Populations of all commercially exploited fish and shellfish are within safe biological limits, exhibiting a population age and size distribution that is indicative of a healthy stock” and “(4) All elements of the marine food webs, to the extent that they are known, occur at normal abundance and diversity and levels capable of ensuring the long-term abundance of the species and the retention of their full reproductive capacity”.

Furthermore, Sweden has the national Environmental Quality Objective ‘A Balanced Marine Environment, Flourishing Coastal Areas and Archipelagos’ and on a global scale the UN Sustainable Development Goal 14 (SDG14)‘Conserve and sustainably use the oceans, seas and marine resources for sustainable development’ is central. All of these management objectives must be respected and the best way of doing that is to implement ecosystem-based management, which stresses the need of allowing various parameters to be included in the management decision and implementation process.

For access to the full research discussed at the seminar and in the article above: