Do aquaculture systems leave a legacy effect?

Insight from a water-treatment system switch indicates that no legacy effects are established in Atlantic cod larvae or their rearing water bacterial communities.
Do aquaculture systems leave a legacy effect?
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In our recent publication, we show that the water treatment system does not establish historical legacy effects in an experimental aquaculture setting. This finding indicates that it continues to be highly important for the aquaculture industry to maintain good microbial rearing water quality throughout the rearing period. 

Marine larvae are extremely vulnerable during their early lives. I was surprised when I learned that the survival rate from egg-hatching to the juvenile stage can be as low as 3% (Vadstein et al., 2018). This vulnerability is, of course, an economic issue for the industry, but it also points to the fact that we lack a comprehensive understanding of how we can rear the fish optimally.

Atlantic cod larvea




















Figure 1: Atlantic cod larvae are small and vulnerable to suboptimal conditions during their first life stages. The photo shows the larvae sampled at 9-, 12- and 20-days post-hatching in 2 mL Eppendorf tubes.

It’s now agreed that the microbial water quality is critical for increasing larval viability. However, the approaches used to obtain good quality vary. For example, many disinfect the water to reduce the bacterial load in an effort to keep the fish healthy. However, as our research groups confirm time after time, it’s the kind of bacteria present around the fish that impacts survival – not the mere presence of bacteria. The “if you can’t beat them – join them” approach has in numerous experiments increased the larval viability.

Different water treatment systems used for treating the incoming water impact what kind of bacterial community that is established in the rearing water. What causes these differences are not fully understood yet – but there is strong evidence that increasing the bacteria’s retention time in the system allows for mature and more stable communities to develop.

We have previously shown that conventional flow-through systems (FTS) select for opportunistic bacteria (Vadstein et al., 2018). This opportunistic growth is caused by a drastic increase in the available recourses when the bacteria go from the inlet water pipes to the rearing water tanks. To reduce the opportunistic growth, our group developed the matured water system (MMS) (Skjermo et al. 1997). In the MMS, the incoming water is cultured in a biofilm tank before it’s delivered to the fish rearing tanks. Thus, the opportunistic growth happens away from the fish, and it is possible to decouple the biofilm- and rearing- tank’s water retention time. This decoupling can further stabilize the water community, enhance bacterial competition, and select for a more beneficial bacterial community. Using MMS has drastically increased the survival of marine larvae, confirming that the type of bacteria present in the system is important. The observation that MMS lead to higher survival leads to the question; Is it the initial bacterial colonization of the larvae that impacts viability, or is it the microbial rearing water quality that affects survival?

Figure 2: Our experimental design to test if the water treatment systems MMS and FTS leave legacy effects in the rearing water tanks. Sixteen rearing tanks were given either FTS or MMS water at high or low resource availability, yielding four groups. After nine days, we switched the inlet pipes in half of the tanks giving them a new water treatment system. We monitored the larval survival and robustness in addition to sampling the rearing water bacterial communities at 1- and 12-days post-hatching (DPH).
Figure 2: Our experimental design to test if the water treatment systems MMS and FTS leave legacy effects in the rearing water tanks. Sixteen rearing tanks were given either FTS or MMS water at high or low resource availability, yielding four groups. After nine days, we switched the inlet pipes in half of the tanks giving them a new water treatment system. We monitored the larval survival and robustness in addition to sampling the rearing water bacterial communities at 1- and 12-days post-hatching (DPH).

We investigated this question by rearing Atlantic cod larvae in tanks receiving water from either a flow-through or a microbially matured system. In addition, we ran the experiment at two different resource levels (Figure 2). After nine days, we switched the inlet pipes to half of the tanks to observe the effects of changing the inlet water on the water’s bacterial communities and the larvae. We were particularly interested in determining if the initial water treatment system left a legacy effect in the larvae or the water.

Legacy effects are caused by conditions that an ecosystem experienced in the past. They are historical, deterministic effects. The idea is that previous settings, in our case, the water treatment system, change the ecosystem in a way that is detectable later. Our experiment was designed to compare if the initial water treatment system resulted in a legacy effect. Our investigation focused on two biological components of the rearing system; the larvae and the bacterial communities.

Madeleine Gundersen amplicon library preparation
Figure 3: The bacterial communities of the rearing water were collected on a 0.2 um filter. The cells on this filter were lysed, and the DNA was extracted. Next, I amplified the 16s rRNA gene of the extracted DNA and sequenced it using Illumina MiSeq sequencing. This method allows us to estimate which bacteria are present in the system and at what relative abundances we find them.

In brief, we found no indication that the initial rearing system left legacy effects by comparing the switched and unswitched rearing tanks. Instead, the larvae and their rearing tank bacterial communities were most impacted by what kind of water system they were experiencing and the amount of available resources in the water.

Moreover, we again confirmed that high bacterial loads might benefit the larvae if managed correctly. In our high-resource MMS systems, we observed high bacterial stability in the rearing water, most likely due to seeding from the MMS biofilm. But for the high-resource FTS tanks, we observed a 90% reduction in survival compared to the high-resource MMS tanks (16% vs 1.5%). These observations hint that lowering the bacterial load (through, e.g., disinfection) can have detrimental effects for the fish. Thus, more research should investigate how disinfection within the rearing systems impacts larval viability, a practice becoming more common in recirculating aquaculture systems (RAS). Furthermore, our findings are important because they illustrate how important it is to properly manage the microbial rearing quality from the start and throughout the rearing period of marine larvae to obtain increased viability.

References:

  1. Vadstein, O., Attramadal, K. J. K., Bakke, I., & Olsen, Y. (2018). K-Selection as Microbial Community Management Strategy: A Method for Improved Viability of Larvae in Aquaculture. Frontiers in Microbiology, 9, 2730. https://doi.org/10.3389/fmicb.2018.02730
  2. Skjermo, J., Salvesen, I., Øie, G., Olsen, Y., & Vadstein, O. (1997). Microbially matured water: A technique for selection of a non-opportunistic bacterial flora in water that may improve performance of marine larvae. Aquaculture International, 5, 13–28. https://doi.org/10.1007/BF02764784

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