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Noble Crayfish (Astacus astacus) in a Changing World: Implications for Management
Mid Sweden University, Faculty of Science, Technology and Media, Department of applied science and design.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The noble crayfish (Astacus astacus) is critically endangered in Sweden. This is mainly due to the crayfish plague (Aphanomyces astaci), a lethal disease that, among other things, can be spread through the stocking of fish from contaminated water or contaminated fishing gear. The largest single propagation path is the illegal introduction of infected signal crayfish (Pacifastacus leniusculus). A conservation measure for crayfish is to re-introduce it to where it has a chance to survive, though a sustainable, locally regulated fishing can also serve as an indirect protection for the species. When the local inhabitants are allowed to keep their fishing culture and when fishing is acceptable, the incentive for illegal stocking of signal crayfish is low. However, it is important to avoid overfishing because the recovery, especially in the northern regions, can take several years. Therefore, it is important to know how crayfish respond long-term to fishing and environmental factors.

Crayfish populations became extinct in the River Ljungan for unknown reasons in 1999. The water flow of the river has been used for activities such as fishing, timber transport and hydroelectric power since the 1500s, and the noble crayfish has been part of the fauna since the last century. The River Ljungan was known as one of Sweden's best fishing areas for crayfish and fishing became an important part of the local tradition. When the crayfish populations became extinct, a reintroduction program was a natural step, and crayfish are nowadays re-established in the river.

From 1963 to 1990 the Swedish Board of Fisheries collected data from crayfish fishing in the River Ljungan to determine the economic damage to fishery owners caused by the construction of a power plant. After each season the fishermen reported the catch. In this thesis, the data was used to investigate which factors influence the long-term size of the crayfish catch and how the crayfish catches were affected by the power plant building. After re-introduction of the crayfish to the River Ljungan, the local fishermen monitored the population development in a simple, standardized way. To examine the validity of their measurements and to investigate the body growth of the individuals, a capture-recapture technique with a permanent marking of the crayfish was used.

The crayfish catches were primarily impacted by the previous years' catch size, and a large catch the previous year resulted in a reduced catch the following year. A mild winter climate (NAO-index > -0.7) six years before the catch implied a large catch, whereas a high water flow during the autumn or spring (>95m3s-1) two years before the catch, implied a poor catch. Major habitat changes in the form of greatly reduced water flow (~90%) were negative for crayfish catches. The standardized method of fishing used by the local fishermen to monitor the development of the crayfish population was precise enough to detect population trends and this method can therefore be recommended to monitor future re-introductions of crayfish. Although the River Ljungan is located at the northern edge of the species' range, noble crayfish in the river presently have a body growth rate that is close to the maximum measured for crayfish (8 mm/moult for females and 10 mm/moult for males).

Based on the results, the most important advice for sustainable fisheries in Ljungan and other northern rivers is to:

  • Monitor the population trends, NAO-index and water flow in May and October. 
  • Use the results from the monitoring to determine the number of allowed fishing days and traps.
  • Collect data about the catch size and efforts from legal fishing and use it to evaluate the sustainability of the fishing.
  • Enhance the buildup of the harvestable cohort by

-saving reproductive females

-introduce a size limit of 10 cm

-provide proper shelters for the non-harvestable cohort.

Abstract [sv]

Flodkräftan (Astacus astacus) är akut hotad i Sverige främst på grund av kräftpest (Aphanomyces astaci). Kräftpest är en dödlig sjukdom som bland annat kan spridas vid fiskutsättningar från smittade vatten eller med smittade fiskeredskap. Den enskilt största spridningsvägen är illegala introduktioner av smittade signalkräftor (Pacifastacus leniusculus). En bevarandeåtgärd för flodkräfta är att återintroducera den till lokaler där den har chans att överleva, men ett hållbart fiske med lokal styrning kan också fungera som ett indirekt skydd för arten. När lokalbefolkning tillåts att behålla sin fiskekultur och fisket är bra, blir incitamentet för illegal inplantering av signalkräfta lågt. Men det är viktigt att undvika överfiske då återhämtning, speciellt i nordlig miljö kan ta åtskilliga år. Därför är det betydelsefullt att veta hur flodkräftan svarar på fiske och omgivningsfaktorer i det långa loppet.

Flodkräftbeståndet dog, av okänd anledning, ut i Ljungan 1999. I Ljungans flöde har det fiskats, flottats timmer och utvunnits vattenkraft etc. sedan 1600-talet och under det senaste århundradet har flodkräftan varit en del av Ljungans fauna. På sin tid var det en av Sveriges bästa lokaler för flodkräfta. Kräftfisket blev en viktig del av den lokala traditionen, så när kräftbeståndet dog ut var återintroduktion en självklarhet och flodkräftan har åter sin hemvist i älven.

Under perioden 1963 till och med 1990 samlade Fiskeriverket in data från kräftfisket i Ljungan för att fastställa den ekonomiska skadan som fiskerättsägarna åsamkats i samband med ett kraftverksbygge. Efter varje säsong fick fiskarna rapportera hur fisket gått. I den här avhandlingen, har det materialet använts för att undersöka vilka faktorer som påverkar kräftfångstens storlek på lång sikt och hur kräftfångsterna påverkades av kraftverksutbyggnaden. Efter återintroduktionen av flodkräfta till Ljungan mättes beståndsutvecklingen på ett enkelt, men standardiserat sätt av de lokala fiskevårdsområdena. För att undersöka validiteten av deras mätningar och hur kräftornas individuella utveckling fortskred, användes fångst- och återfångstteknik, med permanent märkning av kräftorna.

Kräftfångsternas storlek påverkades främst av tidigare års fångststorlek; en stor fångst föregående år innebar minskad fångst följande år. Ett milt vinterklimat (NAO-index > -0.7) sex år före fångst innebar bättre fångster, medan höga vattenflöden höst och vår (>95m3s-1) två år före fångsttillfället innebar sämre fångst. Större habitatförändringar i form av kraftigt reducerade vattenflöden (~90%) var negativt för kräftfångsterna. Den standardiserade metoden som fiskevårdsområdena använt för att mäta beståndsutvecklingen var tillräckligt precis för att påvisa populationsutvecklingen och kan därför rekommenderas också för uppföljning av andra återintroduktioner av flodkräfta. Trots att Ljungan ligger i norra kanten av flodkräftans utbredningsområde har flodkräftorna i Ljungan för närvarande en kroppstillväxt som är nära den maximala som uppmätts för flodkräfta (8 mm/ömsning för honor och 10 mm/ömsning för hannar).

Utifrån resultaten är de viktigaste råden för ett hållbart fiske i Ljungan och andra nordliga vattendrag att:

  • Övervaka kräftstammens utveckling, NAO-index samt vattenflöde i maj och oktober. 
  • Använda resultaten från övervakningen för att bestämma antalet tillåtna fiskedagar och burar.
  • Samla in data om fångststorlek och hur många burnätter som faktiskt gjordes under säsongen. Använda data för att utvärdera fiskets hållbarhet.
  • Stärka uppbyggnaden av den fångstbara storleken genom att

-spara reproduktiva honor

-införa en storleksgräns på 10 cm

-tillse att det finns gömslen för kräftor av icke-fångstbar storlek.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2012. , 40 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 126
Keyword [en]
Noble crayfish (Astacus astacus), CPUE, climate change, water regulation, body growth, reintroduction, sustainable fishing, monitoring.
National Category
Natural Sciences Ecology
Identifiers
URN: urn:nbn:se:miun:diva-16313ISBN: 978-91-87103-17-9 (print)OAI: oai:DiVA.org:miun-16313DiVA: diva2:528661
Public defence
2012-05-25, Mittuniversitetet, sal L 111, Holmgatan 10, Sundsvall, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2012-05-28 Created: 2012-05-28 Last updated: 2015-07-01Bibliographically approved
List of papers
1. Time series analysis of climate-related factors and their impact on a red-listed noble crayfish population in northern Sweden
Open this publication in new window or tab >>Time series analysis of climate-related factors and their impact on a red-listed noble crayfish population in northern Sweden
2012 (English)In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 57, no 5, 1031-1041 p.Article in journal (Refereed) Published
Abstract [en]

1. Global climate change is predicted to raise water temperatures and alter flow regimes in northern river systems. Climate-related factors might have profound impacts on survival, reproduction and distribution of freshwater species such as red-listed noble crayfish (Astacus astacus) in its northern limit of distribution.2. In this study, noble crayfish capture data over 27 years from the River Ljungan, Sweden, were examined. Time series of catch per unit effort (CPUE) were analysed in relation to the North Atlantic Oscillation (NAO) index, regional weather factors and water flow. CPUE was assumed to reflect differences in population size. Two models were constructed to explore the relative impact of different climate factors and density dependence on variability of catch sizes.3. The most parsimonious model for CPUE time series, explaining 72% of the variance in CPUE, included density-dependent population dynamics of the crayfish and climate or weather factors. The specific effect from density dependence in the model was 37%, while climate/weather factors contributed with 35% of the variation. The most important climate/weather factors are variations in NAO index and water flow. Temperature did not improve the model fit to capture data.4. The best model was evaluated using independent data sets that gave correlations between model predictions and data ranging from 0.44 to 0.53. The density dependence shows a time lag of 1 year, while climate variables show time lags from 2 to 6 years in relation to CPUE, indicating effects on different cohorts of the crayfish population.5. Both density dependence and climatic factors play a significant role in population fluctuations of noble crayfish. A 6-year time lag for NAO index is puzzling but indicates that some as yet unidentified factors related to NAO might act on the juvenile stages of the population. Water flow shows a 2-year lag to the CPUE, and high flow in the river may affect adult survival. The reasons for fluctuation of crayfish catches in response to climate need to be identified, and fishing quotas should consider the different cohort sizes because of variation in environment. Reintroduction programmes for crayfish need to consider effects of climate change when designing management strategies.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keyword
catch per unit effort, crayfish, north Atlantic oscillation index, time series, water flow
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-16144 (URN)10.1111/j.1365-2427.2012.02764.x (DOI)000302397300012 ()2-s2.0-84859431789 (Scopus ID)
Available from: 2012-04-28 Created: 2012-04-28 Last updated: 2017-12-07Bibliographically approved
2. Influence of water regulation and water flow on Noble crayfish (Astacus astacus) catch yield in Ljungan a Northern Swedish River after building of a power plant
Open this publication in new window or tab >>Influence of water regulation and water flow on Noble crayfish (Astacus astacus) catch yield in Ljungan a Northern Swedish River after building of a power plant
2010 (English)In: Freshwater Crayfish, ISSN 2076-4324, Vol. 17, 141-144 p.Article in journal (Refereed) Published
Abstract [en]

Free-flowing water is thought to be of importance for reproduction and body growth of noble crayfish (Astacus astacus (Linné)) in the northern edge of its distribution area.

A hydroelectric power plant with a bypass tunnel was built in the river Ljungan in 1976.  This reduced the mean water flow in the old river bed from about 60m3/s to 3m3/s. The sites with the largest reduction of water flow had the largest decline in catch per unit effort of crayfish.  At one site the catches were reduced by >60%, but the catch success varied between locations.  In all sites the regulation seemed to have an effect, even though catch per unit effort was possibly affected by a number of other factors.

KEY WORDS: Catch yield, noble crayfish (Astacus astacus), water flow, water level regulation

Keyword
Catch yeld, noble crayfish, water flow, water level regulation
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-8172 (URN)
Available from: 2009-01-09 Created: 2009-01-09 Last updated: 2013-02-12Bibliographically approved
3. Reliability of catch per unit effort (CPUE) for evaluation of reintroduction programs – A comparison of mark-recapture method with standardized trapping
Open this publication in new window or tab >>Reliability of catch per unit effort (CPUE) for evaluation of reintroduction programs – A comparison of mark-recapture method with standardized trapping
2011 (English)In: Knowledge and Management of Aquatic Ecosystems, ISSN 1961-9502, E-ISSN 1961-9502, no 401, Art no. 7- p.Article in journal (Refereed) Published
Abstract [en]

Catch per unit effort (CPUE) is used as a standardized trapping method by local fishermen and in monitoring studies. In this study, CPUE was compared with population estimates made with a capture-recapture method based on the passive integrated transponder (PIT-tag) marking of individuals. The results show a stronger positive correlation between the estimated population sizes from the capture-recapture method with an estimated CPUE effort of 120 traps. The fishermen used 15 traps, and even this effort showed a fair correlation with the mark-recapture estimates. This indicates that the standardized way of trapping with 15 traps can be used to evaluate reintroduction programs and monitor crayfish populations

Keyword
Astacus astacus, CPUE, capture-recapture, reintroduction, evaluation
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-12554 (URN)10.1051/kmae/2011016 (DOI)000297214300007 ()2-s2.0-79956332728 (Scopus ID)
Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2016-10-26Bibliographically approved
4. Body growth of noble crayfish (Astacus astacus) after reintroduction to a northern river (Ljungan, Sweden)
Open this publication in new window or tab >>Body growth of noble crayfish (Astacus astacus) after reintroduction to a northern river (Ljungan, Sweden)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Noble crayfish were introduced to the River Ljungan as part of a reintroduction program. Crayfish were caught at five sites and individually marked with passive integrated transponders (PIT-tags). At each sampling occasion, the crayfish were sexed and their total lengths measured before being released to the same spot where they were captured. The size structure and growth rates were compared between the different sites and a linear growth model was developed from recapture data. The analysis revealed that crayfish moulted once annually with length increments close to the maximum that is reported for the species. The relative growth increment of males was 20% greater than for females. The growth and size of the length increment were not associated to different habitats or water temperature at the sites. Growth was also measured in the River Ljungan in the 1960s. When compared to the conditions from the 1960s, it could not be precluded whether the differences in climate or population density from that time had a positive effect on the body size growth rate as observed today. A conclusion from this study is that noble crayfish have ability to grow just fast as at its southern locations as the northern edge of its distribution area.

Keyword
body growth, length increment, Noble crayfish, PIT-tags, reintroduction
National Category
Ecology
Identifiers
urn:nbn:se:miun:diva-16145 (URN)
Available from: 2012-04-28 Created: 2012-04-28 Last updated: 2012-07-26Bibliographically approved

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