Background
On 20-23 November 1995, the Swedish Environmental Protection Agency (SEPA)
hosted the Second Workshop on Reproduction Disturbances in Fish, in Lidingö,
Stockholm. The workshop was financed jointly by SEPA, the Swedish National Board
of Fisheries, the Swedish Council for Forestry and Agricultural Research, the
World Wide Fund for Nature, the Swedish Power Board and the Nordic Council of
Ministers. The Lidingö workshop featured presentations by invited speakers,
plenary lectures, posters and group discussions including the following areas:
- The Early Mortality Syndrome (EMS) and Swim-Up Syndrome (SUS) in
North American salmonids - The M74 syndrome in Baltic salmon (Salmo solar) -
Recruitment problems in Baltic cod (Gadus morhua) - Reproduction problems in
other Baltic fish species
The workshop was attended by over 70 scientists from 10 countries,
representing expertise in disciplines such as environmental chemistry and
toxicology, molecular biology, population genetics, fisheries ecology, fish
physiology, pathology and parasitology, aquaculture and environmental
protection.
M74/EMS in salmonids Early Mortality Syndrome may be regarded as the
collective name for reproductive disturbanses observed during the last 20 years
in salmonids from the Baltic Sea and the North American Great Lakes. The M74
syndrome in Baltic salmon and the Cayuga syndrome in Atlantic salmon (Salmo
salar) should be considered as special cases of EMS. Pathological and
behavioural signs of the syndromes were demonstrated at the workshop by several
oral presentations (Jenny Lundström, Susan Marcquenski, Jeffrey Fisher) and
by the two videofilms that documented similarities between the M74 syndrome
(Per-Åke Hägerroth) and EMS (Susan Marcquenski). Similar clinical
signs in newly hatched yolk-sac fry of affected salmonids include loss of
coordination, irregular swimming patterns, hyperactivity, darkening of the skin
and high mortality. In Baltic salmon, some adults display abnormal swimming
behaviour ("wiggling").
In the North American Great Lakes, recent outbreaks of the Early
Mortality Syndrome affect several salmonid species, including coho salmon
(Oncorhynchus kisutch), chinook salmon (Oncorhynchus tshawytscha), steelhead
salmon (Oncorhynchus mykiss), lake trout (Salvelinus namaycush) and brown trout
(Salmo trutta). Most of the clinical signs are similar among the different
species (Susan Marcquenski). Since 1993, the onset of clinical signs in yolk-sac
fry has begun at a slightly earlier developmental stage compared to EMS in
previous years. The main factors thought to be responsible for EMS are changes
in ecosystem structure and function (e.g. introduction of non-indigenous fish
species and changes in the abundance of prey fish) and nutritional factors (e.g.
low levels of thiamine (vitamin B1) in salmonid eggs).
The Cayuga syndrome in Atlantic salmon from the New York Finger Lakes
demonstrates similarities with M74 in Baltic salmon (l. e. both are
thiamine-related neurological syndromes) and offers particularly good
opportunities for parallel research and comparison with M74, e.g. comparison of
pathological and behavioural symptoms and the role of environmental factors
(Jeffrey Fisher, Jenny Lundström).
There are a number of ecological similarities associated with the
reproductive disturbances in the North American salmonids and the M74 syndrome
in Baltic salmon, e.g. the syndromes affect newly hatched yolk-sac fry and have
a rapid onset, development of the syndromes is female-dependt, affected fry are
responsive to thiamine treatment, and food webs in both ecosystems are based on
clupeid fish (Report from working group 3). Another common factor is the
presence of many non-indigeneous species in the Baltic Sea and the North
American Great Lakes, for instance the zebra mussel Dreissena polymorpha, which
has become established in both regions.
The M74 syndrome is still occurring at high levels in Swedish salmon
hatcheries, where reproduction is based on wild-living broodstock that are
predominantly of hatchery origin ( the proportion of wild salmon in the Baltic
is less than 20% of the whole stock). During the past four years (1992-1995), 57
to 87% of the female spawners in hatcheries produced offspring that were
affected by M74 (Bengt-Erik Bengtsson). Wild salmon stocks can be assumed to
suffer from similar losses, as fry hatched from eggs of wild spawners that are
brought into hatcheries also experience high mortality from M74, and
electrofishing surveys in rivers without salmon hatcheries indicate a low
abundance of part relative to the number of adults that migrated upriver to
spawn.
Data from Sweden and Finland indicate that sea trout (Salmo trutta)
develop a disease simililar M74 (Antti Soivio). Finnish scientists have also
been able to demonstrate that thiamine is effective for treating affected fry of
sea trout (Perttu Koski). From these observations we may assume that M74 now
also occurs in several populations of sea trout, and that current reproductive
disturbances in salmonids should be monitored carefully all around the Baltic
Sea. As part of this monitoring work, it is very important to separate the eggs
and progeny of individual female broodfish, as progeny from different females
display different mortality rates.
Since the Uppsala workshop on reproduction disturbances in fish, which
was held in October 1993 (Norrgren 1994), there has been a considerable increase
in research efforts on the cause of these disturbances, both in the Baltic and
in the Great Lakes regions. The Uppsala workshop launched the "carotenoid/astaxanthin
hypothesis" (presented by Alce Lignell and to-workers, Lignell 1994) as a
possible explanation for the M74 syndrome (salmon fry with low levels of
carotenoids showed a high incidence of M74).
If we were to try and identify the major fmding from the Lidingo
workshop, it is probably that both M74 in Baltic salmon and EMS and SUS (Swim-up
Syndrome) in Great Lakes salmonids can be treated with thiamine. Positive
results of such treatment were reported from Canada (John Fitzsimons), the
United States (Jeffrey Fisher, Michael Hornung), Sweden (Patric Amcoff, kim
Larsson) and Finland (Perttu Koski). This points to a similar origin for these
syndromes and considerably narrows the focus of research on the mechanisms
involved.
However, the role of astaxanthin and lipophilic toxic contaminants has
not yet been resolved, and these hypotheses can not yet be ruled out. For
instance, astaxanthin levels are low in fry affected by M74 (Annette
Pettersson), and injection of carotenoid pigments may reduce mortality due to
EMS in steelhead trout (Michael Hornung). Certain persistent pollutants, e.g.
specific organochlorines, may be implicated in M74 (Christoffer Rappe, Pekka
Vuorinen). Pollutants that are known or suspected to be increasing in biota, and
contaminants that are receptor-specific (e.g. hormone-like substances) are
particularly interesting to study (Report from working group 1).
There is still a lack of knowledge about the role of thiamine (and
other vitamins) in fish metabolism and of the dynamics of thiamine in aquatic
ecosystems. This lack of knowledge could be partly addressed by thorough reviews
of the literature. We also lack information about the long-term effects of the
thiamine-treatment practices carried out in the hatcheries today on the health
and survival of fry and their ability to reproduce as adults (Report from
working group 1 ).
Genetic bottlenecks have occurred both in stocks of Baltic salmon and
in chinook and coho salmon in the Great Lakes. There is no direct evidence that
genetic factors are involved in the occurrence of M74 and EMS, but their role
deserves further studies (Torbjorn yon Schantz, Åsa Langefors, Per-Erik
Olsson, Report from working group 2).
Dietary factors, which may result from changes in ecosystems or food
webs, seem to remain one of the most important factors behind reproductive
disturbances in salmonids (Lars Karlsson, Erkki Ikonen, Susan Marcquenski). The
presence/dynamics of clupeid fish species that are eaten by the affected
salmonids also seem to be important. In the Baltic, abiotic factors such as high
water temperatures in winter may also be related to the incidence of M74 (Report
from working group 3).
If we were able to identify the reasons for M74 within the next few
years, and if a treatment could be applied, wild-spawning Baltic salmon
populations would still be at risk. The most effective way of protecting the
wild stocks is to reduce coastal and offshore fisheries on mixed populations,
and reduce releases of reared smolt (Reports from working groups 2 and 3). To
alleviate the effects of M74 on wild-spawning populations, different measures
(e.g. gene banks, finclipping to identify hatchery stocks) must be developed to
enhance their survival and preserve wild stocks (Lars-Ove Eriksson, Jorma
Piironen).
Other species Baltic cod populations are suffering from recruitment
difficulties (Stig Mellergaard, Lennart Balk), but present data do not support
the existence of a syndrome similar to M74 in salmon. Rather, the major
prerequisites for successful spawning in Baltic cod are adequate salinity and
oxygen conditions in the spawning areas (Lars Vallin, Anders Nissling). A
combination of overfishing and poor salinity and oxygen conditions in the 1980s
(due to eutrophication of the Baltic Sea and a lack of influxes of saline water
from the North Sea) is thought to have negatively affected spawning stocks and
larval survival of cod.
There are indications of reproductive disturbances in other species of
fish in the Baltic, but the pathological and behavioural signs are not the same
as those of the M74 syndrome and EMS (Bengtsson et al. 1994). However, there
have so far been very limited efforts to search for reproductive disturbances in
a wide range of fish species, and it is possible that some of the reproductive
disturbances are different manifestations of the same problem. Two contributions
on species other than salmonids and cod were presented at the Lidingö
workshop: a Finnish study on the ability of parasites (microsporidians) to cause
reproductive disorders in roach (Rutilus rutilus) (Goran Bylund); and an
Estonian study on levels of polycyclic aromatic hydrocarbons in eelpout (Zoarces
viviparus) (Risto Tanner).
Conclusions The positive effects of the coordination of research
around the Baltic Sea and cooperation with researchers in the United States and
Canada in this field are very evident, and it was further emphasized by the
participants that cooperation must continue. This process will be served by
coordination of research programmes, exchange of materials and methods, and
financial support to attend workshops and other meetings where ideas and results
can be exchanged. The participans encouraged each other to utilize established
and new networks, communication via Email/Internet and participation in
workshops/symposia for this purpose.
References: Bengtsson, B.-E., A. Bergman, I. Brandt, C. Hill, N.
Johansson, A. Sodergren & J. Thulin (1994).
Reproductive disturbances in Baltic fish. Research programme for the period
1994/95-1998/99. Swedish Environmental Protection Agency Report 4319.
Lignell, Å. (1994).
Astaxanthin in yolk-sac fry from feral Baltic salmon. In: Norrgren, L.
(ed.). Report from the Uppsala workshop on reproduction disturbances in fish.
Swedish Environmental Protection Agency Report 4346, pp. 94-96.
Norrgren, L. (ed.) (1994).
Report from the Uppsala workshop on reproduction disturbances in fish.
Swedish Environmental Protection Agency Report 4346.
|