Prey-predator interactions between the myctophid Bentosema glaciale and calanoid copepods in the Labrador Sea

Prey-predator interactions between the myctophid Bentosema glaciale and calanoid copepods in the Labrador Sea P. Pepin Northwest Atlantic Fisheries Centre

Throughout the North Atlantic, copepods of the genus Calanus are dominant players in the zooplankton community C.finmarchicus is most important both numerically and in terms of integrated production www.sahfos.org

Previous modeling of C.finmarchicus had shown that paramaterization based on conditions in the Eastern Atlantic yielded inaccurate results, particularly in the Labrador Sea Speirs et al. (2006)

Previous modeling of C.finmarchicus had shown that paramaterization based on conditions in the Eastern Atlantic yielded inaccurate results, particularly in the Labrador Sea The introduction of temperature-dependent mortality term yielded more realistic seasonal pattern in the spatial distribution Speirs et al. (2006)

The assumption of a temperature-dependent mortality term in modeling C. finmarchicus appears reasonable but there are few data on which to base this decision To achieve strong predictive capacity requires strong causal relationship Under the assumption that the primary cause of mortality is predation, and with limited knowledge of the sources of mortality in sub-arctic ecosystems, we turned our attention to the potential role of the Deep-Scattering Layer (DSL) Within the region, and other similar ecosystems, a major component of the DSL consists of myctophids, with Benthosema glaciale being a dominant species (Sameoto 1989; Bagoeine et al., 2001) and evidence indicated they could have a significant impact on the zooplankton community

Benthosema glaciale Pelagic Non-migratory Depth range 0-1100 m Planktivorous Max length ~ 10 11 cm High probability of occurrence throughout the North Atlantic www.fishbase.org

Observation program Completed two exploratory surveys of NL Shelf and Labrador Sea (June, August Sept 2006) Zooplankton collections (integrated and vertically stratified samples) to measure condition (lipids), egg production Hydroacoustic surveys to determine distribution of potential predators (planktivorous fish) Stomach sampling program to estimate predator consumption rates (some depth stratification) Focus on deep water and areas of potential cross shelf transport LATITUDE 46 48 50 52 54 56 58 42 43 41 40 26 25 39 38 27 28 29 24 22 23 20 21 1 2 3 37 30 36 31 19 4 5 6 58 56 54 52 50 48 46 44 42 35 18 34 7 LONGITUDE 32 33 17 8 16 9 16 10 14 13 11 12

Observation program

Observation program

Observation program Depth (m) / Profondeur (m) 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 0 30 60 90 120 150 180 210 240 270 300 330 Teleost 673 (10-12 June/Juin 2006) -6-6.5-7 -7.5-8 -8.5-9 -9.5-10 -10.5-11 Log (SA[ m 2 m -2 ]) Depth (m) / Profondeur (m) 0 50 100 150 200 250 300 350 400 450 500 0 30 60 90 120 150 180 210 240 270 300 330 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0-1 -2 Temperature ( o C) Near surface scattering layer present on NL Shelf and in Labrador Sea consists principally of amphipods with some medusae Deep scattering layer (DSL) is present throughout Labrador Sea Average temperature at 400-500 m ~ 4 C

Observation program Deep scattering layer (DSL) 50-70% of biomass consists of myctophids Background integrated Sv in Lab Sea ~ 10 80 times that found on NL Shelf Percentage by weight Pourcentage par poids 100 80 60 40 20 0 Amphipods Hatchet fish Unidentified Balck Giotre Shrimp Viper Boa Lanceet Squid Snipe Eel Myctophids 1000 Integratal SA (m 2 m -2 ) 1e-3 1e-4 1e-5 1e-6 1e-7 0 30 60 90 120 150 180 210 240 270 300 330 100 10 1 0.1 Lumiere a la surface (µmol m -2 s -1 ) Surface light (µmol m -2 s -1 ) Distance along transect (nautical miles) Distance le long du transect (milles nautiques)

Abundance estimates Range of target strength measurements for myctophids MacLennan and Forbes (1987) TS = -61.7 db Torgersen and Kaartvedt (2001) TS (modal) -63-58 db Valinassab (2000) TS = -60-55 db Benoit-Baird and Au (2003) TS = -44.8 db Range of densities ~ 8 53 fish m -2 Range of biomass ~ 15 190 mt km -2

Observation program 0.30 Proportion above 100m 0.25 0.20 0.15 0.10 0.05 Mean (10 th - 90 th percentile) Median Extensive diurnal vertical migration of DSL but only in a fraction of DSL (10-25% of integrated Sv) Difficult to determine whether there is replacement of individuals in upper layer 0.00 0 4 8 12 16 20 24 NDT

Dusk to midnight 0.030 Dusk to midnight 0.5 0.025 0.4 Deep (300-600 m) Surface (< 250 m) GFI ( w stomach / w fish ) 0.020 0.015 0.010 Relative proportion 0.3 0.2 0.005 0.1 0.000 Deep (300-600 m) Surface (<250m) 0.0 <0.001 0.001-0.010.01-0.020.02-0.030.03-0.040.04-0.05 >0.05 GFI ( w stomach / w fish )

Midnight to dawn 0.030 0.5 0.025 0.4 Deep (300-600 m) Surface (< 250 m) GFI ( w stomach / w fish ) 0.020 0.015 0.010 Relative proportion 0.3 0.2 0.005 0.1 0.000 Deep (300-600 m) Surface (<250m) 0.0 <0.001 0.001-0.010.01-0.020.02-0.030.03-0.040.04-0.05 >0.05 GFI ( w stomach / w fish )

Diurnal cycle (300 500 m) 0.06 Vertical migration motivated by hunger 0.05 level GFI ( w stomach / wf ish ) 0.04 0.03 0.02 Relative proportion of migrants suggests that turnover rates ~ 4 8 d 0.01 0.00 00:00-04:00 04:00-08:00 08:00-12:00 12:00-16:00 16:00-20:00 20:00-24:00

20 Prey length (mm) Longeur de la prioe (mm) 10 9 8 7 6 5 4 3 2 1 0.9 0.8 0.7 0.6 0.5 10 20 30 40 50 60 70 80 90 Myctophid length (mm) Longeur du myctophide (mm) CVI CV CIV CIII CII CI CVI CV CIV CIII CII CI CVI CV CIV CIII CII CI C. finmarchicus C.hyperboreus C. glacialis Amphipoda Calanus spp. Calanus fhg Calanus finmarchicus Calanus glacialis Calanus hyperboreus Calanoid copepod Copepoda Euchaeta spp. Euphausiidae Hyperidae Unidentified species

20 Prey length (mm) Longeur de la prioe (mm) 10 9 8 7 6 5 4 3 2 1 0.9 0.8 0.7 0.6 0.5 10 20 30 40 50 60 70 80 90 Myctophid length (mm) Longeur du myctophide (mm) CVI CV CIV CIII CII CI CVI CV CIV CIII CII CI CVI CV CIV CIII CII CI C. finmarchicus C.hyperboreus C. glacialis Amphipoda Calanus spp. Calanus fhg Calanus finmarchicus Calanus glacialis Calanus hyperboreus Calanoid copepod Copepoda Euchaeta spp. Euphausiidae Hyperidae Unidentified species

0.35 Fish length (20-35 mm) 0.5 Fish length (45-55 mm) 0.30 0.4 0.25 Relative frequency 0.20 0.15 0.10 C. glacialis C. finmarchicus C. hyperboreus Relative frequency 0.3 0.2 C. glacialis C. hyperboreus 0.05 0.1 C. finmarchicus 0.00 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Prey (thorax) length (mm) Prey (thorax) length (mm)

Analytical results stomach contents Generalized linear model (Poisson error structure) Number of prey per fish Fish length ( P < 0.001) Time of day ( P < 0.001) [highest after dawn lowest at dusk] Positively affected by temperature (0-100m) ( P < 0.001) Q 10 ~2.2 Temperature effect significant for fish sampled in surface layer and not significant for fish in deep layer General linear model (log-transformed GFI) GFI Time of day ( P < 0.05 ) [highest midnight to midday] [lowest midday to midnight] Negatively affected by temperature (300-500) ( P < 0.001) for fish sampled in deep layer Q 10 ~1 No significant effect of temperature for fish sampled in surface layer

Analytical results stomach contents General linear model (log-transformed prey length) Prey length Significantly positively related to fish length ( P < 0.001) Not significantly affected by temperature ( P > 0.1) Significantly related to time of day ( P < 0.001) [larger prey more frequent in stomach sampled midday to dusk] Significant seasonal (June vs September) effect ( P < 0.001) [probably a reflection of differences in availability few larger copepods in August relative to June]

LATITUDE 46 48 50 52 54 56 58 36 37 38 35 39 40 34 41 42 43 32 33 31 30 29 27 28 16 16 26 17 14 25 18 19 13 20 24 21 22 23 1 2 3 4 5 6 7 58 56 54 52 50 48 46 44 42 LONGITUDE 8 9 12 11 10 Depth (m) 0 100 200 300 Vertical distribution of C. finmarchicus Similar distributions for sister species Density (# m -3 ) 0 500 1000 1500 2000 0 100 200 300 Density (# m -3 ) 100 200 300 400 500 600 400 400 500 500 600 0 500 1000 1500 2000 600 100 200 300 400 500 600

Analytical results Mortality estimate Range of analytical assumptions Fish density ~ 8 53 m -2 Mean number of prey per fish (surface feeders) ~ 2 10 Turnover rate ~ 0.10 0.25 d -1 Proportion of prey C. finmarchicus CIII-CVI ~ 0.6 C. finmarchicus density ~ 10,000 30,000 m -2 Total consumption ~ <1 100 m -2 Mortality 0.00004 0.008 d -1

Summary and Conclusions The deep-scatter layer (DSL) is widely (and almost uniformly) distributed through the western Labrador Sea, with an overall biomass 8 190 mt km -2 On average, 70% of the biomass consists of Benthosema glaciale Preferred temperature range: 3.7 4.1 C Only 10 25% of the DSL migrates to surface waters daily B. glaciale that migrate to the surface appear to be motivated by hunger Digestion time / turnover rate ~ 4 10 days

Summary and Conclusions Calanoid copepods make up 80% of the prey found in the stomachs Late stage C. finmarchicus, C. glacialis and C. hyperboreus are more prevalent in the stomachs than earlier stages of each species In contrast to expectations, measures of feeding activity (number of prey, GFI) are not equally affected by environmental temperature numbers of prey in surface feeders are positively affected by temperature (Q 10 ~ 2.2) GFI of fish in either surface or deep layers is negatively affected by temperature(q 10 ~ 1) Overall impact of B. glaciale on C. finmarchicus results in mortality rates < 1% d -1

Issues for further research Analysis of feeding patterns in B. glaciale requires further assessment Role of hunger in motivating activity and feeding pattern Effect of local prey availability on feeding activity (numbers, biomass, selection) Metabolic activity in cold environments??? Other sources of mortality for Calanus in the region?

~100% Themisto libellula In the Gulf of St. Laurence, A. Marion (University du Quebec, Rimouski) estimated that T. libellula could ingest ~2 % of the mesoplankton community, and exert a predation rate of ~3% d -1 on C. finmarchicus (CIV CVI). B. Glaciale may play a role in dynamics of C. finmarchicus but role of invertebrate predators needs to be investigated (greater overlap, turnover rates, local density)

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