CHALLENGES FOR DANISH INLAND FISHERIES MANAGEMENT, CONFLICTS WITH OTHER WATER USES

Pedersen et al. (2012) and Rasmussen and Pedersen (2018) summarize some of the future challenges for the state of Danish streams. In spite of a strong focus on stream restoration

in recent years, conditions in many places are still far from optimal, with respect to all phases of the salmonid life cycle. Many barriers still exist and channelized streams often do not offer suitable habitats for the young trout. Recent status has indicated that approximately 26 percent of Danish streams (either small entire streams or parts of larger streams) produce less than 50 percent trout than could be expected based on stream capacity (HELCOM, 2011). The reason for this is, in most cases, due to poor habitat conditions (including heavy sand transport) or barriers (e.g. Birnie-Gauvin et al., 2017), including newly established artificial lakes in the lower parts of the streams. These lakes

FIGURE 32

Estimated spawning population of Atlantic salmon in rivers Storå, Skjern and Varde over the years

have been constructed as part of wetland projects established in order to reduce the level of nutrient emission (nitrogen) to coastal areas through denitrification. Such lakes have demonstrated to have a devastating effect on migratory salmonids, resulting in heavy smolt loss during seaward migration (Schwinn et al., 2019), and more projects involving this kind of lake are likely to be planned in future. One potential solution could be to construct artificial lakes with only partial water intake of the main flow (Pedersen et al., 2012). Another challenge is sand erosion, where sand from areas around the streams (fields, roads, urban areas, construction sites, etc.) and heavy sediment transport results in sand covering spawning gravel and reducing habitat availability. Covering of the spawning gravel results in loss of spawning possibilities – severely reduces egg survival – and loss of habitats for young trout (Pedersen et al., 2012).

Predation from cormorants in particular, has been documented to be an important factor for many populations of stationary and migrating river fish and likely also plays a major role in the availability of fish for angling in lakes (Skov et al., 2014) and coastal areas (Jepsen et al., 2010). Since cormorants changed their foraging behaviour and started to visit even small streams, some fish populations have declined and the combination of a large otter and heron population with cormorant visits have had negative impacts on especially brown trout, grayling and salmon populations (Jepsen et al., 2014, 2018, 2019). Management/regulation of cormorants has become more active, with several hundred permissions for regulated control of cormorant populations issued annually, including intensive egg-oiling and hunting along rivers in winter, but so far without measurable effect on the level of predation. For more information consult the EIFAAC cormorant project.

Climate changes are likely to affect both streams and lakes and consequently inland fisheries (Harrod, 2015). Specifically in streams, more precipitation and in turn elevated discharge, is likely to increase maximum flow and hence increase erosion and sediment transport. More precipitation is expected during winter and locally in summer where heavy rainfall in connection with thunderstorms is expected to increase in frequency. Also, longer periods without precipitation are predicted, which can result in a reduction of productive area and consequently reduced production of brown trout and Atlantic salmon smolts. Already today, a number of streams are affected by water extraction for consumption, especially near larger cities and on some of the Danish

FIGURE 33

Estimated spawning population of Atlantic salmon in rivers Storå, Skjern and Varde over the years

islands, and in some places the subsequent reduced minimum flows is compensated by artificially adding water to the streams during critical periods (Pedersen et al., 2012).

Climate changes are also likely to affect lakes in numerous ways, e.g. changes in size distribution of fish and a lower relative share of predator fish (Jeppesen et al., 2012).

Maintenance of streams, such as cutting stream macrophytes, removal of accumulated sediments and removal of woody debris, is regularly undertaken in most streams according to the Watercourse Act. This can have severe impact, particularly on salmonid populations as it can result in lack of shelter and changes in flow patterns. In recent years, there has been focus on introducing maintenance practices that are less harsh, but in many streams, maintenance is still unnecessarily heavy. In the coming water plans, reduced or more lenient maintenance is also proposed as a way to improve riverine habitat quality (Pedersen et al., 2012).

The majority of land in Denmark consists of drained agriculture fields and large areas with solid surface (roads, etc.). This results in rapid run-off through ditches and consequently large fluctuations in discharge, which has a negative impact on salmon and sea trout populations, especially in smaller catchments (Pedersen et al., 2012).

Point emissions from industries and sewage are no longer a general problem but are sometimes observed locally in the upper parts of streams. Accidents with sudden and heavy pollution with organic material from agricultural activity are observed from time to time, either when semi-liquid manure is spread on the fields as fertilizer or because of mishaps at the farms where tonnes of animal manure can end up in streams (Pedersen et al., 2012).

In the past, non-native, commercially-important fish species were often introduced into Danish waters. However today, the Nature Protection Act, stipulates that non-native species must not be introduced and even for introduction of non-native fish species to a watercourse where they did not originally occur, requires special permission. It is known that invasive species can have negative impacts on indigenous species and their ecosystem, and that the zoogeographical diversity is worth preserving. Still, illegal introduction of non-native and native non-local fish and crayfish do occur frequently in lakes and streams (Skov et al., 2011).

Nutrient levels in lakes have been reduced over the last 30 years in many lakes due to reduced external input of nutrients such as via inflowing streams or drainage channels.

However, in many of these lakes, internal loading, i.e. nutrients stored in the sediment, maintains high-nutrient loads. Moreover, heavy summer rainfall sometimes results in overflow at sewage plants and overflow waters with high-nutrient levels can end up in nearby, often urban, lakes. Therefore, eutrophication is still a problem in many Danish lakes (Johansson et al., 2019). Lake restoration attempts are, from time to time, initiated according to the EU WFD.

A few lakes and rivers still suffer from “ghost of the past” pollutions like heavy metals and organic compounds. An example stems from the Grindsted Products factory, which produced medicine and food additives in the 1960s, and resulted in leaching of heavy metals and organic compounds into a nearby lake and rivers. Still today, mercury contents in the flesh of pike in a nearby lake are so high that it is not fit for human consumption. Moreover, vinyl chlorides are still leaching from the area where the factory was located, causing concentrations 40 times higher than allowed in the nearby stream.