Baltic Breakfast: Fisheries regulations and environmental factors behind the strong Riga herring
Limitation on vessel size, closed periods for fishing and beneficial environmental changes. Those are some possible factors behind the favourable development of the herring stock in the Gulf of Riga, according to the researchers participating in the last Baltic Breakfast.
The assessment and management of the herring stocks in the Baltic Sea are divided in four units: Western Baltic herring, Central Baltic herring, Gulf of Bothnia herring and Gulf of Riga herring.
In difference to the other three stocks, the spawning stock biomass (shortened as SSB) of the Gulf of Riga herring has increased for the last decades, to a historical high value.
“The stock is in very good shape”, says Olavi Kaljuste, who is Environmental Monitoring and Assessment Analyst at the Department of Aquatic Resources, Swedish University of Agricultural Sciences (SLU-Aqua).
While the recruitment of the Western Baltic and Central Baltic herring has decreased, it has increased in the Gulf of Riga. The fishing pressure has been too high (above FMSY) for most of the time for the Western Baltic and Central Baltic herring. In the Gulf of Riga, the pressure has also been quite high for part of the period and moderate for the rest, but this has not seemingly affected the spawning stock biomass.
“Somehow this stock tolerates higher fishing pressure than the other stocks”, says Olavi Kaljuste.
Characterisation of the stock
There are several differences between and within the herring stocks in the Baltic Sea, explains Olavi Kaljuste. The open sea herring stocks perform migration between the spawning grounds, which often are located in the gulfs, and the feeding and wintering areas in the Baltic Proper, while the gulf herrings mainly remain in the gulfs.
The Gulf of Riga herring mature at a younger age and smaller size, and they reach the final stage of maturation later in spring or autumn, compared to the Central Baltic herring. Within the Gulf of Riga, the herring in this stock spawn about a month later than the Central Baltic herring individuals that enter there to spawn.
“The Gulf of Riga herring overwinter in colder water, so there is longer fasting period during that time”, says Olavi Kaljuste. “Therefore, the maturation of reproductive cells is depending on the feeding in the spring, so they need to feed first, before they can spawn.”
Two natural types of herring
There are two broad types of herring in the Baltic Sea: the spring spawners and the autumn spawners. The Gulf of Riga is dominated by the spring spawners, only 1 percent is autumn spawners.
“There was a time at the 1940-50s when the share of autumn spawners was higher: close to 50 percent. But the climate changed and there were big shifts in the Baltic in temperature and salinity, and the autumn spawners disappeared.”
The Gulf of Riga herring is slow growing with one of the lowest mean length and weight at age of the Baltic stocks. This is due to the late spawning time and the shorter feeding season, tells Olavi Kaljuste. The stock can also easily be distinguished from the central Baltic herring by the structure of the otoliths (a bone in the ear), and by its different body shape.
Importance of management
Olavi Kaljuste points out that there is a mismatch between the management units of the herring in the Baltic Sea and the population structure. The species is distributed all over the Baltic forming several populations which are adapted to the local conditions, while the management only operate with four stocks.
“The Gulf of Riga herring is the only management unit where there is in practise one local population dominating”, says Olavi Kaljuste. “The other management units include several stock components which are mixed with neighbouring stocks.”
The management of such combined population faces several difficulties, and the assessments may not provide an adequate picture of the developments.
“Therefore, applying the natural population structure as a basis of assessment and catch advice can prevent the critical decrease in abundance and/or the collapse of some exploited populations”, says Olavi Kaljuste.
Different ecosystem
Ivars Putnis, at the Institute of Food Safety, Animal Health and Environment (BIOR) in Latvia has also been doing research on the Guld of Riga herring and the management of it.
“The Gulf of Riga herring is a famous looking fish in the Baltic Sea – it’s the herring with the big head and the slim body”, he says.
The Gulf of Riga ecosystem is also quite different than the rest of the Baltic Sea. The gulf is shallow, brackish as it is influenced by the river runout and also quite productive. The shallow water means that the food web reacts fast to changes in the environment.
“There were some times when cod was abundant in the Gulf of Riga, but nowadays the main boss in the system is herring, and the main source of commercial fisheries as well”, says Ivars Putnis.
In the 1980s, the herring in the Gulf of Riga were bigger than today, but the size dropped in the beginning of the 1990s.
“This goes together with a common regime shift in the central Baltic Sea. In the Gulf of Riga, there was mainly a change in the zoo plankton composition. The energetically important food item Limnocalanus has almost gone when the water temperatures became warmer.”
The herring stock also increased in the beginning of the 90s, which led to higher intraspecific competition, says Ivars Putnis. In the recent years, however, the size has been more or less stable.
Fisheries regulations
The management of the Gulf of Riga herring include some specific regulations. One is that the engine power of a vessel must not exceed 221 kW at any time, and the total engine power is also limited.
There are also several trawling restrictions in force: the vertical opening of the trawl cannot exceed 12 meters and no demersal trawling is allowed. Trawling in areas shallower than 20 meters is not allowed and there are also closed seasons: 30 days in spring in both Latvia and Estonia, and in Estonia an additional trawling ban is in force from mid-June to mid-September.
“My main conclusions would be that the Gulf of Riga herring has benefited from recent environmental changes and regime shift”, says Ivars Putnis. “Specific fisheries management is in force with the aim to protect pre-spawning shoals and to limit vessel power to limit the fishing intensity.”
Furthermore, it is also beneficial that only two countries are involved in the stock assessment and fisheries, means Ivars Putnis. There are few target species in the Gulf of Riga and the herring caught is mainly used for human consumption. The sampling of the commercial landings is intensive and the scientific staff working with the assessments have long-term experience.
What of these specific fisheries measures would you recommend other managers? asks one person in the audience.
“One key point could be to look more in detail on the population structure”, says Ivars Putnis. “A lot of recent research suggest that there are many sub populations in the Baltic area that could react differently on environmental pressures.”
Olavi Kaljuste adds that the herring in the Gulf of Bothnia is quite similar to the herring in the Gulf of Riga and could possibly managed in the same way.
“But the biggest difference for me in the Gulf of Riga is that all these measures are made to protect the pre-spawning shoals – not to allow trawlers to go and fish when the fish is easiest to catch; when they are schooling”, he says. “It’s also a big difference in how the quotas are given. In for example Estonia, they are split almost 50-50 between the trawl fisheries and the coastal fisheries. So almost 50 percent of the fish is taken by coastal fishermen when the fish is spawning. But it is okey because they fish with trap nets and the fish is spawning within the trap nets, so there is recruitment coming from that too.
How important is the regulation on the vessel power?
“This is a good point and it could probably be a recommendation too”, says Ivars Putnis. “The regulation of the power regulates the size of the trawl, so it is probably crucial in this fragile ecosystem.”
“This is actually very important”, says Olavi Kaljuste. “This limitation makes trawl fishing spread all over the year, which is good because the composition of fish changes. With huge gear you can fish the quota fast, but that hurts the stock. In the Gulf of Riga, it is spread out and I think that is better.”
Text: Lisa Bergqvist
See a recording of the seminar here:
Last updated: November 28, 2023
Source: Baltic Sea Centre