Do fish larvae control their own destiny?

(Written by Nadia Fouzai)

Figure1. Schematic overview of the computer model describing basic environmental gradients, with vertical temperature and light gradients, variable food distribution and predation risk. Yellow arrows indicate larval movement in the water column.

Most fish live through a dangerous larval period after the protected embryonic phase inside the eggshell and before they transform into a robust juvenile fish. During this critical period, the larva lives of the lunch bag it got from mother - the yolk sac – but soon it is empty and the larva must find and catch its own small plankton. To survive this period is a big challenge for fish larvae. Their success depends on how they navigate in the surrounding environment, and utilize every possible loophole. The water column may seem like a smooth and homogeneous place, but it has strong spatial and temporal gradients of temperature, light, food and predators.

Most Norwegians have a special interest in cod, the skrei, the most valuable wild fish in Norway, feeding in the Barents Sea and spawning along the Norwegian coast. In late winter and early spring, skrei migrate several hundred kilometers from their feeding areas to spawn at specific grounds. Their strategy is to find the most suitable environment for their offspring - ensure that their first self-sustained feeding event is successful so they have a good start in life. But what if the ocean environment conditions are not perfect? Do larvae have the ability to react to information from their environment and make the best out of the conditions they are given? Which strategies and decisions give them the best chance to survive? Can they influence their own destiny, even if they are so small and vulnerable? To answer this question, we need a computer model.

Optimal larval cod behaviour in vertical environmental gradients

We have built a computer model to find out how much influence larval cod has on its own destiny. What can a larval cod do to improve its chances in life by moving around in gradients of light, temperature and food supply?  When should they take a chance and fill their stomach by actively searching for prey, and when should they sit still and hide in the deep dark?

Technically, we offer the larva a surrounding environment with warmer water at the surface, light decaying with depth and varying over the course of the day, time of year, latitude, cloud cover and the clarity of the water, variable food distributed over the water column and predation risk from fish and invertebrates (Fig 1). Then, we follow a population of fish larvae in four different ocean environments, growing from first feeding until it is 15 mm long. The larvae find the best immediate habitat and swimming activity, from fear and hunger, opportunities and risks. From chance and necessity, most die, but some survive and grow and become skrei.

Larval fish need to eat to grow and survive; they have to swim actively and be in water with enough light to see to find the small plankton they feed on. However, when they are eating, they also expose themselves to predators and, since they are in the light, large predatory fish can see them. What can fish larvae do to find food, grow fast and at the same time avoid predators? Their toolbox is limited; they can only regulate their swimming activity and depth choice.  To make things even more complicated if they swim much to find food or migrate up and down in the water to hide from fish predators, they increase the chance of running into predators that are sitting still and waiting for prey to pass by. For instance, jellyfish have dangerous stingy, sticky tentacles, and the torpedo-shaped arrow worms are hanging around with sensory hairs to detect small vibrations in the water from small fish passing by.  The pelagic zone is a jungle, but if the larva is a chicken and hides too much, the stomach runs empty, it grow slowly, and have a longer time in the vulnerable larval stage. Tough trade-offs are everywhere.

Now, what will happen to all of these trades-offs when temperatures and food availability change? Does larval life become easier? We use an algorithm called dynamic programming to solve the questions of life and depth for the larva.  The best thing to do the next hour depends on where it is in space, how full the stomach is, its body size, the time of day and all environmental settings. With the model, we can find the very best way to live a life as a larva, the exact choices to do to maximize chance of becoming a skrei. The model allows us to examine the nature of the trade-offs, and predict the best actions that animals should make.

Can small fish solve such complicated mathematical problems, you may ask, probably not. Larval cod are not supersmart computernerds, but some fish eggs do pass through the larval stage and eventually produce new eggs. It is not random which larvae make it through; it is those that respond to internal and external cues in ways that promote survival. The mechanism is natural selection and evolution. Therefore, we think that assuming larvae are quite smart is closer to reality than to assume they are ignorant and passive, with no cards to play.

Up or down, sit or go, fear or greed?

So what did we find? Not surprisingly, we found that increasing temperature lead to faster growth and higher larval survival, but only if there is sufficient food. Higher temperatures combined with low plankton abundance are detrimental for larvae, not because of the risk of dying from starvation, but because they have higher metabolism and need more food in warmer water.

The extra food needed has a predation cost. Similarly, the success of the larva increase much if there is more prey available. The surprising prediction here is that the success will continue far beyond the prey abundance where they do not grow any faster. Because finding food is dangerous, they can spend less time feeding and more time hiding when prey is abundant. It just makes larval life much easier - while the monstrous arrow worms get less skrei in their diet.

Figure2 shows detailed depth selection of small larvae (6 mm) and large larvae (10 mm) over 24 h cycles.

When we ran the computer model for four different ocean environments of temperatures and food supply, predictions show that larvae tend to vary their responses to different environmental stimuli (Fig 2). When they are small, less visible to fish predators, the best strategy to optimize their chance in life is to remain close to the surface.

There, larvae can have access to more food, grow fast and have less time in the vulnerable larval stage. Choices change when they grow larger and become more visible to fish predators. Their strategy is to undertake diel vertical migrations between deeper waters with less light and fewer predators during the day, and shallow waters in the photic zone at night, where their food source lives.

By moving up, larvae can forage safely during dawn and dusk and fill their stomach to grow, and by retreating to deeper water during day, they can significantly decrease their risk of predation. Surprisingly, larvae in warm waters choose to take a more risky behaviour; they make an extra trip to the surface around midday to have a supplementary meal. Here maintaining high growth is prioritized over the risk of dying from predators.

Larvae fish can actually affect their own destiny by vertical migration, which provides them with conditions favorable to growth and survival. The conventional wisdom is that the fate of each larva depends entirely on the surrounding environment. Model predictions, however, have shown that small changes in where the larva positions itself vertically can have a dramatic impact on their growth and survival.

Scientific Literature:

Fouzai, N., Opdal, A.F., Jørgensen, C. and Fiksen, Ø. 2015.  Effects of temperature and food availability on larval cod survival: a model for behaviour in vertical gradients. Marine Ecology-Progress Series. 529: 199-212. [ doi:10.3354/meps11326 ] [ open access ]

Larval fish in environmental gradients

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