Do we influence how young fish find home?

Human impacts on the ocean may go even further than you know, disrupting the ability of fish larvae to find their way to a safe habitat…

Thanks to the impact of humans on the ocean environment, we may be altering the way that fishes smell, see and hear in the water. These changes may interfere with the way young larval fish use these senses to find their way to a habitat on which to settle. Although often not considered, it seems that humans may be impacting on larval fish senses in complex ways likely to affect fish populations worldwide.

After hatching from their eggs, many marine fish species must survive a larval phase in deeper offshore waters where they develop and disperse before they are ready to adopt a new home. Even at this early stage many fishes have well-developed sensory abilities, and for a fish larva to survive its journey it needs to use these senses to provide directional information to guide its swimming. Remember, however, that this information is transmitted through water and any big changes being made to that water may disrupt the larva’s chances of successfully locating favourable habitat.

Fishes rely heavily on an acute sense of smell to locate food, avoid predators, communicate and navigate. Changes in ocean chemistry due to pollutants such as pesticides and heavy metals have been shown to cause “olfactory toxicity” in fish, whereby their natural ability to correctly discern the smell of chemical cues has been greatly impaired. The acidification of our oceans due to climate change also has the capacity to disrupt neural processing of smells important to larval fish resulting in changes to innate attraction and avoidance behaviour towards smells from different habitats and predators!

Visual cues are also important to fish larvae. Pollution that alters the available light spectrum in the water column can actually alter the way a fish larva’s eye develops. This can result in changes to spectral sensitivity of the eye that impacts the ability to perceive polarised light. Polarised light, a component of sunlight, is thought to play an important role in providing fish with a type of “compass” which can assist navigation. Even if their eyes develop normally, lower visibility due to higher levels of sediment and nutrients can make it difficult for a larva to find its way.

Aside from sight and smell, the ability of fish larvae to hear sounds from habitat (e.g. the characteristic crunching of corallivorous organisms on coral reefs) can be used to as a way to determine the direction in which to find a place to settle. Here again however, humans have found a way to make things harder for young fish to use their impressive sensory abilities. Sound pollution from common human activities can seriously affect hearing ability or cause damage to ear structures in adult fish, which suggests that the developing senses of larval fish may suffer consequences at least as severe. And as with the other senses, even for fish with their hearing systems intact, sounds from habitat can be “masked” by manmade sound that overwhelms natural sounds the larva is listening out for.

Fish larvae are active animals that depend on their senses to survive and find a new home, a process that is largely responsible for dispersal and connectivity of fish populations. As we do more research on these senses we see that the effects of human activities on the ocean can disrupt or damage these senses in many ways, however we still know very little about the extent of these impacts on fishes.

Perhaps if only one of the many senses fish use were affected the chances for adaptions would be higher, however it seems that all senses are vulnerable to these effects. In light of other threats including over-fishing, habitat degradation and climate change, understanding these impacts on young fish, before they are fully developed and residing in different habitats, is of critical importance.

Jack O’Connor
PhD Candidate, University of Technology Sydney & AMRI

More information:

• Siebeck UE, O'Connor J, Braun C, Leis JM (2015). Do human activities influence survival and orientation abilities of larval fishes in the ocean? Integrative Zoology, 10:65-82.
• Research funded by ARC Discovery grant #DP110100695 (Chief investigators: Dr Jeff Leis & Ulrike Siebeck).