Fish stock analysis involves estimating the size and health of fish populations to understand the impacts of fishing and environmental changes. By analyzing stock data, we can identify trends such as whether a species is increasing or decreasing in an area, or if there are shifts in the age distribution or overall health of the fish. This process helps us determine whether new management measures are needed or assess the effectiveness of previous interventions.
How we study fish and shellfish
To conduct stock analyses of fish and shellfish, we begin by collecting essential data. This data allows us to model stock development and assess sustainable catch levels. We provide advice to authorities regarding the abundance of fish in different areas, including whether populations are increasing or decreasing. Additionally, we evaluate sustainable fishing levels and assess the effectiveness of various protection measures.
Data collection methods
To obtain a comprehensive understanding of fish and shellfish stocks, we employ a range of complementary methods. The choice of method depends on factors such as the specific location and the species being studied
Our data collection methods include:
- standardized field surveys,
- age analysis,
- stereo video systems (filming fish),
- fish tagging,
- fishing statistics,
- acoustic surveys,
- theoretical models.
Standardized surveys: nets and fyke nets along the coast
One of the most traditional methods for assessing fish stocks along the coast is to use a standardized gillnet sampling. Generally, nets are used for sampling on the east coast, while fyke nets are more commonly used on the Swedish west coast.
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The nets are typically left out overnight. The following morning, we collect the catch, identify the species, and record the count, weight, and measurements of each individual.
The goal is to evaluate the size of the fish stocks and track changes over time. To ensure consistency and enable comparisons across years, we conduct the surveys at the same time of the year, in the same locations, and with the same equipment each time.
Age Analysis
Stock estimates rely heavily on age data, making it crucial to determine the age of the fish. The most common method is to remove the fish's otoliths and analyze the natural growth rings (similar to those found in trees). Otoliths are the earbones of a fish and are present in pairs. For certain fish species, we can also determine age by examining the operculum (that covers the gills) and the scales. Additionally, we may take samples to assess whether the fish have accumulated environmental toxins.
Filming fish with stereo video systems - BRUV
Some areas, like the shallow offshore banks of the Kattegat, are challenging to access and study. The rocky seabeds make it difficult to use traditional fishing gear, such as nets or trawls. Since 2018, we have been exploring a new method—filming the fish. This approach allows us to observe the fish without disturbing them or the environment. It also enables us to study areas that are otherwise difficult to reach with trawls or nets.
The stereo video system we use is called BRUV (Baited Remote Underwater Stereo-Video System), which consists of two cameras placed with bait in front to attract the fish. We lower the cameras to the seabed and allow them to film for an hour before retrieving them. Once back on land, we analyze the footage using specialized software that helps us measure the fish's length to the nearest centimeter.
However, BRUV is not as precise as trawling and other fishing methods and cannot be used in conditions with turbid water or strong currents.
