How does a fish stock survey work and why is it important?
Fish stock surveys in the Netherlands: insights into biodiversity and ecological water management
What is a fish stock survey?
Fish stock surveys are used to gain insight into the composition and health of the fish population in Dutch inland waters. Municipalities, sports fishing associations and water boards use the results to make responsible decisions about water management, fish stocking and ecological monitoring. A healthy fish population says a lot about water quality and the ecosystem as a whole. Now that the growing season is ending and carp are actively gathering food for the winter, September is the time to conduct fish stock surveys; the results then provide a good overview of recent trends as well as possible bottlenecks.
Who conducts a fish stock survey?
Research is usually conducted by professional fishermen in cooperation with ecologists. They have permits and expertise to catch, analyze and release fish in an animal-friendly manner. The combination of ecological knowledge and practical fishing techniques makes the research reliable and effective.
How does a fish stock survey work?
A fish stock survey begins with choosing an appropriate trapping method. In smaller waters such as ditches and ponds, electric fishing is often used. This method temporarily stuns fish with an electric current, after which they can be safely measured and surveyed. In larger waters, one uses a seine, a large trawl that effectively fishes larger areas.
What exactly are researchers measuring?
Data collected during a fish stock survey include fish species composition, length, weight, age composition, sex and sometimes fish condition or disease status. This information provides insight into the ecological balance of the water body.
Why is fish stock research important?
Fish stock surveys are essential for sustainable water management. It helps identify ecological problems such as water pollution, oxygen deficiency or overgrowth. It also provides insight into the effectiveness of measures taken earlier. Up-to-date fish data is also indispensable in dredging projects and the redesign of banks.
Electric fishing fixed on image
A short video in which during a fishing survey with an electric fishing device the necessary large bream are caught. This was filmed with the drone, so you can clearly see the big fish swimming in front of the boat. The fish were measured after being caught and returned to the same water.
The similarity to koi fish
For koi enthusiasts, seine fishing is recognizable from draining mud ponds. In natural waters, this technique is more complex because of obstacles and varying depths. Nevertheless, it remains a valuable method for getting a broad overview of the fish population.
Special catches and biodiversity
Special catches are regularly made during fish stock surveys. Consider rare native species or the presence of invasive exotic species such as sunfish or blackbeaked goby. These catches sometimes lead to additional research or policy measures.
Want to know more?
On the Sportvisserij Nederland website you will find regular updates on notable catches and ongoing investigations. Visit Sportvisserij Nederland.
Remarkable carp and fish stocks in recent years
Based on recent reports and trends in the Netherlands, we can make a few observations about carp and fish stocks:
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Increase of carp in canals due to stocking
A significant increase of carp has been observed in certain canals. This is largely attributed to stocking by sport fishermen, as carp hardly reproduce in many Dutch waters. -
Concern about excessive carp numbers
Carp can cause bottom dwelling (turbidity), which affects aquatic plants and thus water quality. Management agencies are reluctant to allow large carp releases. -
Relationship between nutrient richness (nutrients) and dominance of carp & bream
In waters with high nutrient richness, the proportion of carp and bream increases. Sometimes these species become dominant, which can have ecological consequences. -
Sport fishery releases remain important but declining in part
The number of carp releases has been less consistent in recent years, in part due to stricter regulations and restraint by water boards. -
Changes in juvenile fish and vegetation associated
Fish stock surveys (such as at the Marker Wadden) show that juvenile fish (0+ year classes) are highly visible in September, and the relationship with vegetation and habitat structure determines how successful spawning and survival are. Carp juveniles (young carp) are often more difficult to find outside of suitable habitats.
Resources
Rijkswaterstaat – Development of fish stocks in the Netherlands
Sportvisserij Nederland – Carp in the Netherlands
Waterschap Zuiderzeeland – Fish stock survey Vollenhover and Kadoelermeer (2022).
