Between 2014 and 2022, fish surveys were conducted in 153 water bodies. Each water body was fished at least once during this period. Larger bodies of water, especially those requiring regular updates under the WFD, underwent multiple surveys at different sampling points.
Electric fishing was the main technique used. Fishing followed approved methodological principles, typically using direct current (DC) generators. Portable, battery-operated devices were used for wadeable ditches and small water bodies, while larger ones were surveyed from boats equipped with more powerful generator-operated devices. DC generators create an electric field in the water, which influences fish based on their length and position relative to the field lines. Fish responses can range from fleeing to positive electrotaxis (swimming toward the anode) or even electronarcosis, depending on the voltage they encounter. The effective capture range of the electrode is about 2 metres.
When implemented correctly, electrofishing is the least disruptive method for surveying fish populations because it minimises contact with netting and similar materials used in other methods. With electrofishing, fish sustain minimal injuries to scales or mucous membranes. It is also the most effective method for fishing in rough soil substrates featuring rocks, riprap, or vegetation. As a result, electrofishing has become the standard method for assessing fish populations under the WFD in flowing waters.
The length of each fishing section ranged from 300 to 3,000 metres of shoreline, depending on the width of the water body, the complexity of structures, and the fishing success rate. The goal was to obtain the most comprehensive picture of the species range possible. However, pelagic (open water) species and large, elusive individuals of all fish species are often underrepresented in electrofishing. These fish can evade the effective capture field because their flight distance exceeds the anode’s range, resulting in them being caught only incidentally. Overall, electrofishing has a much lower selectivity compared to other methods, even when similar effort and time are invested.
In addition to electrofishing, multi-mesh gillnets were used in groundwater-fed lakes to catch fish swimming in open water, which are methodologically underrepresented in electrofishing. Gillnets operate on the principle that a fish attempting to swim through the net becomes trapped (gilled) in the mesh, enclosing its body tightly enough to prevent escape. The nets capture fish best when their body circumference is about 25 % larger than the mesh size, which only applies to a narrow range of fish lengths. Gillnets therefore have a high degree of size selectivity. The chosen mesh size determines both the size range of fish and, to some extent, the range of species that can be caught.
Multi-mesh nets are made up of panels with varying mesh sizes, allowing them to capture a broader range of fish sizes compared to nets with a uniform mesh size. In the open-water surveys of Berlin’s groundwater-fed lakes, 30-metre-long and 1.5-metre-high multi-mesh bottom-set gillnets were employed. The number of nets used was adjusted based on the size of each water body. Each net consisted of 12 panels, with mesh sizes of 5, 6.25, 8, 10, 12.5, 15.5, 19.5, 24, 29, 35, 43 and 55 millimetres. To minimise harm to the fish, the nets were deployed for a maximum of two hours.
In fluvial lakes and large flowing waters, the Fischereiamt Berlin (Fisheries Office) also used trawls. The IBG (Institute of Freshwater Ecology and Inland Fisheries) did the same in the Großer Müggelsee. Similar to gillnetting, this approach aimed to survey open-water fish species, which are often under-represented in electrofishing surveys. Unlike gillnets, which only catch fish when they try to swim through them, trawls are actively pulled through the water, also capturing inactive fish. While trawls are better suited for assessing the abundance of open-water fish, they require significantly more technical effort compared to gillnets.
In August 2004, the Fischereiamt Berlin launched the research vessel ‘PISCATOR’, which has since been regularly used for trawling, among other activities. A trawl is a large net bag with wings, towed behind the boat and kept open by trawl doors. The mesh size of the net bag (the ‘cod-end’) determines the shortest length of the fish caught, with little size selectivity beyond that. The Fischereiamt used various trawls of different designs and mesh sizes in the net bag. The trawl lengths varied between a few hundred metres and two to three kilometres. Every two years, the IGB uses the research vessel ‘Paulus Schiemenz’, launched in 2011, for trawling as part of the fish monitoring at the Müggelsee.
Using various types of devices helps account for differences in selectivity, making the recordings more representative of the actual fish stocks and, in particular, the species present. These surveys were supplemented by qualitative insights from commercial fishers, who shared observations, such as a notable catch of a sea trout in the Unterhavel on 15 February 2017. Additionally, data from the Stiftung Naturschutz (Foundation for Nature Conservation) provided valuable information on unusual by-catches, primarily non-native species like the goldfish and the pumpkinseed.
All catches are typically analysed by identifying, counting, and measuring all captured fish, along with weighing random samples. When large numbers of fish from a specific age or size group are encountered, a representative sample is measured, while the remaining specimens are only counted. This approach minimises handling time and allows for faster release, reducing stress on the fish.
The present analysis examines the fishery data from the Fischereiamt Berlin with a focus on water bodies. An additional brochure, featuring an up-to-date analysis specifically centred on fish species, will be published in 2024.