ICES CM 2013/F:02
Beneficial influence of microalgae and protists on early stages of herring (Clupea
harengus) larvae.
Björn Illing1, Marta Moyano1 and Myron A. Peck1
Institute of Hydrobiology and Fisheries Science, University of Hamburg, Hamburg Germany. Presenter
contact details: [email protected], Phone +49 40 42838 6708
1
Summary
Based on the microbial loop concept, the trophic link between protozooplankton and ichthyoplankton
has recently gained increased attention. We conducted two laboratory experiments to examine the
effects of microalgae (Rhodomonas baltica, RB) and protists (Oxyrrhis marina, OX) on 0-16 days posthatch (dph) larvae of spring-spawned Baltic herring (Clupea harengus). Larvae were reared (Exp 1) in
Clear Water (CW) or RB+OX, with no additional prey and (Exp 2) in CW, RB or RB+OX, in the
presence of natural prey (Acartia tonsa nauplii). In both experiments, condition (RNA-DNA ratio),
swimming activity and trypsin activity were measured. On each day of Exp 1, groups of 20 larvae
were introduced to new tanks with nauplii for 4h and swimming activity and gut contents were
examined. Results from Exp 1 suggested that larvae benefited from the presence of microalgae and
protists by exhibiting precocious (2 days earlier) feeding as well as higher feeding activity (up to 60 %
more feeding strikes) compared to the CW larvae. Nevertheless, survival was not enhanced in the
RB+OX treatment. When nauplii were continuously present (Exp 2), larvae in the RB and RB+OX
treatments grew significantly faster (1.6 times greater dry mass in RB+OX), were in a better condition
(25 % higher RNA-DNA ratio) and more active (⅔ shorter pause durations) than CW larvae by 16
dph. These results highlight the important link that exists between phytoplankton/microzooplankton
and the timing and magnitude of early feeding and the nutritional condition and growth of larvae
within natural (meso-zooplankton) prey fields.
Introduction
Many laboratory studies have reported that the presence of microalgae increases survival of firstfeeding larvae (e.g. Reitan et al., 1997). This “green water” technique benefits fish larvae in several
ways: it 1) gives a stronger contrast to prey particles; 2) constitutes a direct supply of nutrients; 3)
stimulates the larvae’s appetite and digestive enzyme activity; and 4) establishes an early gut
microbial flora by affecting the bacterial populations in the rearing water. In addition to microalgae,
protists have been also reported to play an important role in the change from endogenous to
exogenous feeding, the “protozooplankton-ichthyoplankton link” (Montagnes et al., 2010). Besides
upgrading larval prey quality, protists can also act as a direct prey source for small larvae (e.g. Fukami
et al., 1999). Although most protozoans are transparent and seem to be invisible for marine fish larvae
many laboratory studies have shown that larvae can see protists, prey on them on purpose and use
them as a main food source (e.g. Figueiredo et al., 2007). While feeding on protists is energetically
more costly than feeding on metazoans (Hunt von Herbing et al., 2001), the positive effects of
microalgae and protists often outweigh that effect and lead to precocious and intensified feeding with
a larger “window of opportunity” (Overton et al., 2010), which is the period between first-feeding and
the point of no return, when larvae are too weak to feed offered prey and survive. In this study we
explored the effects of algae and protists on growth, condition, digestive activity and behavior of firstfeeding Atlantic herring larvae.
Materials and Methods
Herring from the Southwest Baltic Sea were caught in spring 2012 and the freshly hatched larvae were
randomly transferred to the new experimental tanks (1200 larvae per tank). Two parallel sets of
experiments were conducted: In Exp 1 (50l, 0-14 dph) larvae were reared under two feeding
treatments (CW and RB+OX), each in triplicate tanks and with no additional prey. For Exp 2 (90l, 0-16
dph), larvae were reared at three feeding treatments (CW, RB and RB+OX) in single tanks. Beginning
at day 1, natural prey (Acartia tonsa nauplii) was added only in Exp 2. Abiotic parameters in the flowthrough tanks during the experiments were (means ± SD): temperature (9.64 ± 0.17 °C); salinity (16.4 ±
0.3 psu) and oxygen (9.4 ± 0.1 mg/ml). NH4+ concentrations were always kept below 0.1 mg l-1.
Concentrations of RB and OX, nominally at 10.000 and 1.000 cells ml-1, respectively, were measured
using a coulter counter every morning and adjusted as needed. In both experiments, 20 larvae were
individually sampled per day and tank for investigating 1) growth and condition (RNA-DNA ratio),
and 2) digestive capacity (trypsin activity). Additionally, to assess swimming activity and gut
contents, groups of 20 starving larvae were taken daily in Exp 1, confronted with prey for 4h and later
preserved in formalin. In Exp 2, swimming activity was measured directly in the rearing tanks,
precluding individual capture and gut content analysis.
Results and Discussion
Results from Exp 1 suggested that starved larvae benefited from the presence of microalgae and
protists by exhibiting precocious feeding (2 days earlier: 2 vs. 4 dph) compared to the CW larvae. Peak
feeding was observed at 8 dph. Nevertheless, survival was not enhanced in the RB+OX treatment.
Similar results were found with first-feeding cod larvae, where larvae in water with Nannochloropsis
sp. and OX initialized feeding 2 days earlier at 10 °C than in control groups with CW (Overton et al.,
2010).
When nauplii were continuously present (Exp 2), larvae in the RB and RB+OX treatments grew
significantly faster (1.6 times greater dry mass in RB+OX), exhibited better condition (25 % higher
RNA-DNA ratio) and were more active (⅔ shorter pause durations) than CW larvae by 16 dph. From
day 5, larvae from treatments with algae showed intensified feeding and performed 60 % more
feeding strikes and attempts per min at day 16 than CW larvae. Also, larvae in fed RB+OX treatments
depleted their yolk-sac significantly faster than larvae in RO, CW or all unfed treatments, presumably
due to higher costs of increased feeding activity.
In summary, the presence of algae and protists enhances early feeding and thus higher growth and
condition in Atlantic herring larvae. Our results suggest that fish larvae can benefit substantially not
only from algae, but also from heterotrophic protists that can trophically upgrade the nutritional
quality of algae (such as O.marina; Veloza et al., 2005). In the light of these results it seems even more
important for herring in the field not only to match zoo- but also algal blooms in spring for successful
first-feeding.
References
Figueiredo, G.M., Nash, R.D.M., Montagnes, D.J.S., 2007. Do protozoa contribute significantly to the diet of larval
fish in the Irish Sea? Journal of the Marine Biological Association of the UK 87, 843.
Fukami, K., Watanabe, A., Fujita, S., Yamaoka, K., Nishijima, T., 1999. Predation on naked protozoan
microzooplankton by fish larvae. Marine Ecology Progress Series 185, 285–291.
Hunt von Herbing, I., Gallager, S.M., Halteman, W., 2001. Metabolic costs of pursuit and attack in early larval
Atlantic cod. Marine Ecology Progress Series 216, 201–212.
Montagnes, D.J.S., Dower, J.F., Figueiredo, G.M., 2010. The Protozooplankton-Ichthyoplankton Trophic Link: An
Overlooked Aspect of Aquatic Food Webs. Journal of Eukaryotic Microbiology.
Overton, J., Meyer, S., Støttrup, J., Peck, M., 2010. Role of heterotrophic protists in first feeding by cod (Gadus
morhua) larvae. Mar. Ecol. Prog. Ser. 410, 197–204.
Reitan, K.I., Rainuzzo, J.R., Řie, G., Olsen, Y., 1997. A review of the nutritional effects of algae in marine fish
larvae. Aquaculture 155, 207–221.
Veloza, A.J., Chu, F.-L.E., Tang, K.W., 2005. Trophic modification of essential fatty acids by heterotrophic protists
and its effects on the fatty acid composition of the copepod Acartia tonsa. Marine Biology 148, 779–788.