Marvel at Larval - An Appreciation for Young and Developing Fish

If you’ve ever spent time in, around, or on the water, you likely appreciate its beauty, recognize its mysterious nature, and respect its importance as a resource. Maybe you enjoy SCUBA diving in vibrant reef habitat, snorkeling in a river, fishing from a rock jetty, or just turning over rocks in a stream. If so, then you’ve inevitably witnessed one of water’s most magical creatures: fish!

Fish are beautiful specimens, and I’ve been lucky enough to swim alongside them. They also can be tasty, and I’ve fried many a fresh caught brook trout over a fire while camping. Fish are frisky and quick and I constantly struggle to corral them in a net, attempt to restrain them on a measuring board, or demand a friend snap a picture as I desperately grasp at wiggling slime. We are fascinated with fish of all species, color, and behavior. But let us take a moment to appreciate size. Small size.

Fig. 1 Struggling to tame a lake trout (Salvelinus namaycush) caught during research on Jackson Lake, Wyoming. These larger fish were caught and tagged with tracking devices to help scientists understand reproduction patterns and to identify critical habitat to protect. Photo credit: Diana Miller

It is compelling to think about the journey and the story of survival each fish must face entering the world. Fish are of course not born as behemoth beauties. They experience dangerous challenges from the very beginning of life such as predation, changes in water temperature, and learning to eat and swim. Fishermen and women dream of that large bruiser Salmon on the end of their line, and television loves to show footage of the Great White. Large predatory fish are spectacular, they create a sense of awe, and often give me goosebumps. I once had a friend who almost fell out of the boat while jumping for joy after catching and releasing a 1,000lb, 9-foot blue Marlin. Big fish are appealing to our eyes and trigger adrenaline, but let’s explore the odyssey of a fish through stages of its development, from a tiny egg to a mature adult.

Ichthyoplankton (ik-thee-o-plangk-tuh n) are the eggs and the larvae (immature form) of fish. These early life stage organisms, unable to swim on their own, use current drift to disperse themselves [1]. Most begin by feeding endogenously (or from within), using what is dubbed a “yolk-sac”. You can think of this as the “lunch box” for a fish that provides nutrition essential for development.

Fig. 2 Newly hatched larval fish carrying yolk sac. Photo credit: http://www.magnetictimes.com/index.php?a=68&pic=998

Larvae eventually eat up and absorb this “lunch box” and become exogenous consumers (feeding externally), eating various forms of tiny crustaceans and fish. During this new phase, fish are capable of searching out food for themselves: an important step towards surviving in the world. Soon fins take shape, muscles develop, and scales form. Their locomotion struggles almost behind them, fish begin to propel themselves and take life by the…shall we say, fins!? First, larvae establish an ability to swim vertically through the water column in short spurts. Next, they start to swim horizontally and over greater distances. This represents yet another critical step towards survival as active feeding habits begin [2].

Fig. 3 Larval rainbow smelt (Osmerus mordax) captured in Winnisquam Lake, New Hampshire. Note translucent body, lack of scales and fins, and developing air bladder. Photo credit: Matt Devine

As key components of a fish’s body continue to develop, they become juveniles. Some species stay in their nursery habitat, while others begin to form schools and migrate, exploring other waters. These fortunate individuals are now on their way, seeking adulthood and sexual maturity. But not all are so lucky. For some species, most individuals don’t even survive the larval stage. In fact, massive natural mortality during the first phases of development occur in many fishes. Often times mortality is a result of predation, poor water quality, or competition for resources. A poor understanding of mortality can mean uncertainty in estimating how many fish there are total [3], making it difficult to manage the population and implement fishing regulations.

But don’t be mistaken. Life as a larval fish however is tough, and the struggle is real. They are hatched into an ominous world filled with danger and uncertainty. They are small, slow, and susceptible to environmental conditions, starvation, and predation. Eggs and larvae immediately become prey items to just about everything in the aquatic environment [4]! Unable to swim and no real ‘game plan’ for survival, these fish rely on their various arrangements of color, body parts, and shape to carry them forward. Size and growth rates have been linked to higher probabilities of survival [5], so for fish larvae, there appears to be incentive in the form of selection pressure to grow fast. Fast growth can mean stronger swimming ability, predator evasion, and selective feeding. And it means less time in the larval stage, reducing chances of being eaten.

Larval fish ecology is fascinating and complex. The very best scientific minds in the world are tirelessly uncovering new information, testing hypotheses, and trying to make sense of the mechanisms that drive larval fish survival. Piecing together the puzzle of young fish survival will help protect and promote naturally reproducing populations. So next time you’re out on the water, take a minute to marvel at the struggle for life going on beneath the surface. When you land that next trophy in the net, or witness the slow take of a mayfly from a sipping trout, don’t forget to applaud that fish, for its journey to your hand or camera was remarkable.

References

[1] Kendall Jr, A. 1984. Early life history stages of fishes and their characters. Ontogeny and Systematics of Fishes 11-22.http://ci.nii.ac.jp/naid/10010414274/

[2] Yúfera, M. and M. Darias. 2007. The onset of exogenous feeding in marine fish larvae. Aquaculture 268: 53-63.https://www.researchgate.net/profile/Maria_Darias/publication/223437843_The_onset_of_exogenous_feeding_in_marine_fish_larvae/links/54f3b8be0cf24eb8794c59f1.pdf

[3] Hjort, J. 1926. Fluctuations in the year classes of important food fishes. Journal Du Conseil 5-38. http://icesjms.oxfordjournals.org/content/1/1/5.extract

[4] Yen, J. 1987. Predation by a carnivorous marine copepod, Euchaeta norvegica boeck, on eggs and larvae of the north Atlantic cod Gadus morhua. Journal of Experimental Marine Biology and Ecology 112:283-96. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.210.7122&rep=rep1&type=pdf

[5] Garrido, S., R. Ben-Hamadou, A. M. Santos, S. Ferreira, M. A. Teodosio, U. Cotano, X. Irigoien, M. A. Peck, E. Saiz, and P. Re. 2015. Born small, die young: Intrinsic, size-selective mortality in marine larval fish. Scientific Reports 5:17065.http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4657020/

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