Impact of D. geminata presence on juvenile Atlantic salmon : A project overview

Transcription

1 Impact of D. geminata presence on juvenile Atlantic salmon : A project overview Carole-Anne Gillis, B.Sc. Biology Ph.D. Student at INRS Eau Terre Environnement Supervisors: Normand E. Bergeron Max L. Bothwell March 12 th, 2013

2 Context and Rationale In 2006, mysterious biomass reported in Matapedia River Fishing activities disrupted Blooms were identified as : Didymosphenia geminata (didymo or Rock snot) Reported for the first time in eastern North America (Gillis & Chalifour, 2010) 2

3 Context and Rationale Newly introduced species? Spread to other catchments? Impact on salmonid production? Habitat alteration? Strain on local economy? (Bothwell & Spaulding, 2008) Lack of knowledge of potential effects on Atlantic salmon: 1) Post-hoc studies inconclusive 2) Extent to which didymo impacts habitat and ecosystem function has yet to be determined 3) Lack of knowledge of underlying mechanisms 3

4 D. geminata Mat-forming diatom Cells Tubes Cells secrete tubes from apical porefield Native range Bloom : Polysaccharide stalk material comprises bulk of biomass Robert Chabot Dense, fibrous mats can effectively cover the entire river channel Mat thickness varies (Spaulding & Elwell, 2007; Kilroy, 2004) 4

5 Potential effect on trophic dynamics Since didymo may alter the aquatic food base Higher trophic levels may be impacted (Biggs, 2000; Larned et al., 2007) Effects of didymo on trophic dynamics may vary with : Extent of coverage Life history of fish Mat thickness Feeding ecology Bloom duration (Bothwell & Spaulding, 2008)

6 Study Area 6

7 D. geminata s effect on secondary producers Macro-invertebrate community sampling effort in pre-didymo and post-didymo-affected sites (Gillis & Chalifour, 2010) % Chironomid % EPT Khi 2 ; p < 0,0001 * Absence Presence * * Didymo presence alters BMI community structure Didymo presence increases overvall BMI densities

8 Potential effects on prey Greater BMI densities (Gillis & Chalifour, 2010) Higher benthic densities may result in greater drift (James, 2011) Dense fibrous mats may limit drift and entrainment of prey (Shearer et al., 2007) Previous studies have no replication and efforts insufficient to assess effect of didymo on drift dynamics Smaller taxa Increased densities of small taxa Reduced energy source for fish compared to larger taxa (Shearer et al., 2007) 8

9 Potential effects on foraging behaviour Since the macrobenthic community composition is altered and drifting prey availability may be reduced Juvenile Atlantic Salmon (JAS) foraging behaviour may be impacted (Nislow et al., 1998; Dill, 1983) Drift foray vs. Benthic foray Energetic costs (Godin & Rangeley, 1989) Adapted from Godin & Rengeley, 1989 No existing studies on foraging behaviour in relation to didymo

10 Methods Standardized methods of quantifying the extent of didymo SCI : Standing Crop Index (Kilroy & Bothwell, 2012) Example of contrasting SCI values: SCI = thickness X % cover SCI = 105 SCI = % x 7mm. 95% x 20mm 10

11 JAS Foraging Behaviour Site : Patapedia River: affected by didymo since sites with contrasting didymo SCI (0-850) Methods : Analysis: Focal observations while snorkelling of young-of-the-year JAS Observation period : 15 min Foraging behaviours Prey availability Habitat characteristics were then measured: Flow, depth, SCI, t & substrate Proportion of drift vs. benthic forays Foray rates (Dionne et Dodson 2002) 1.5m (Nislow et al. 1998) 11

12 Site: Prey availability sampling Patapedia behaviour sites Objective: Correlate with observed behaviours Drift: Drift nets (600µm) were set 24h prior to snorkelling observations Drift density calculated (Smock, 1996) Benthos: Collected following drift net retrieval Sample collection: A total of 21 drift nets and benthos samples were collected

13 Benthic Forays (%) Foraging Behaviour Proportion of benthic forays as a function of didymo cover N = 53 6 sites P < Standing Crop Index 13

14 Benthic Forays (%) Foraging Behaviour Proportion of benthic forays as a function of didymo cover N = 53 6 sites P < Standing Crop Index 14

15 Benthic Forays (%) Foraging Behaviour Proportion of benthic forays as a function of didymo cover N = 53 6 sites P < Standing Crop Index 15

16 Drifting prey dynamics Drift densities as a surrogate for prey availability Drift densities compared control site with didymo affected site (Mean SCI = 430) Sub-sample : 2 sites Short-term behavioural response to spatial and temporal resource availability? More data to come 16

17 Potential effects on habitat selection Fish tend to optimize overall energetic gain (Fausch, 1984) JAS feeding habitat preference is defined by : (Heggenes, 1996 ) Prey availability Foraging costs If prey availability is altered by didymo presence (Godin & Rangeley, 1989) Behavioural plasticity Profitability: Favourable foraging areas? JAS habitat selection may also be modified 17

18 Habitat Selection Site: Milnikek River - 6 sites with contrasting SCI values (0-1500) Objective: Verify if JAS tend to avoid or prefer didymo-affected areas Methods: Tagged fish relocated using portable antenna (6x) Habitat availability assessed randomly across site using grid plot (n=25) Habitat characteristics measured at each relocation: PIT-Tag #, location, velocity, depth, t, substrate size, distance from bank, cover and SCI End of season recapture of monitored fish (89%) 18

19 What are the underlying mechanisms? Behavioural plasticity : JAS tend to be resilient to increased didymo SCI over time Critical threshold of didymo extensiveness? Net energy gain sufficient? JAS adjust their feeding strategy in response to habitat change (i.e. Didymo presence) What drives the observed shift in foraging modes? Drifting prey availability Location of prey and profitability

20 CONCLUSIONS & FUTURE WORK Project provides insights to the role of didymo on JAS future in eastern Canada Investigation of unexplored areas of didymo impact research: Foraging behavior Habitat selection Upcoming work: Complete laboratory and statistical analysis for prey availability Apply bioenergetics model to behavioural study to identify net profitable energy in relation to didymo (Booker et al,. 2005) Habitat use analysis Growth rates, condition & stable isotope analysis 20

21 Acknowledgments INRS travel grant Supervisors Field assistants Volunteers RSC