This blog post is provided by Nicole Yee and Nick Brown tell the #StoryBehindThePaper for the paper “Consequences of nest site selection vary along a tidal gradient”, which was recently published in Journal of Animal Ecology.

Parenting is a balancing act—making sacrifices is a daily reality as parents strive to raise healthy children. While the needs of parents and their children can match up, all parents eventually face choices that pit their own well-being against that of their children. This is the dilemma confronting a marine fish species, the plainfin midshipman (Porichthys notatus), as they choose a site to nest each spring.

Male plainfin midshipman, called guarders, migrate from the deep Pacific Ocean to the intertidal zone on the seashore each spring for the breeding season. They clear out sand and sediments from beneath large rocks to make nests. They then attempt to attract females to their nest by emitting long, low frequency “hums.” Securing these nest sites, though, is no easy feat—males compete vigorously to obtain and defend their nest sites.

All of this competition takes place in the ever-changing intertidal zone. As the tide comes in, the nesting area is submerged in water. However, as the tide recedes, the nests become uncovered. Plainfin midshipman are often left stranded in small pools of water in their nests where oxygen levels deplete quickly until, eventually, the males are completely exposed to open air. The intertidal zone forms a gradient of environmental conditions: in higher elevation areas closer to the shore, fluctuations in water levels and temperature are more frequent and extreme.

It is under these challenging conditions that plainfin midshipman raise their young. Fathers spend about two months in their nests guarding their eggs from predators, a period during which they do not eat.

So how does parenting differ between males nesting at lower elevations, further from the shore where conditions are calmer and more stable, versus males nesting at higher elevations in the intertidal zone where conditions are more extreme?

To answer this question, we needed to observe plainfin midshipman raising their young in the wild. In 2018, our team discovered that some males chose to nest in the subtidal zone, which is always underwater.  We managed to study the development rates of eggs in these different zones because, unlike the fish themselves, eggs cannot swim away and stay stuck to the rock under which they were laid. After uncovering some interesting differences in development rates between the intertidal and subtidal nests, we were compelled to look deeper into subtidal-nesting fish in 2019.

Our biggest challenge was to observe and capture these fish without scaring them away. Because of the 4-meter tidal range at our research site, water depths could change by almost 1 meter per hour during the most extreme tides. So, we reasoned that SCUBA diving would be impractical and opted instead to use free (breath hold) diving to monitor the fish.

Free diving allowed us to move gently around the nests in a range of water depths. Working together, we gently uncovered nests to capture males for measurement. In other nests, we inserted small video cameras to film behavioural trials.

Snorkelling and free diving proved an elegant strategy for working in an environment where water depths change constantly and dramatically. It was imperative to our work that we cause as little disruption as possible to the fish.

Looking at videos of males in their nests when submerged allowed us to measure their behaviours. We initially predicted that males nesting higher in the intertidal zone might have less energy to provide parental care. We reasoned that just existing in this environment, where air exposure can be a daily occurrence, would take a toll on the health of these fish. To our surprise, we found that males at higher elevations on the beach actually spent the most time cleaning their nests, fanning water across their eggs, and engaging in other parental care behaviours. The recorded observations revealed that when the conditions are tough, males spent more time providing care. Perhaps, then, this increased care is meant to meet the additional needs of their young, despite the costs to themselves.

We also found that eggs laid at higher elevations in the intertidal zone developed much faster than those at lower elevations, likely because warmer water and air temperature accelerate development.

You might be thinking; surely, the young raised higher in the intertidal zone, that had more parental attention and developed faster would survive in higher numbers. To your surprise and ours, we found the opposite. Despite their fathers’ best efforts, offspring in the higher intertidal died at faster rates compared to those in the lower tidal regions. Low in the intertidal, we saw that offspring survival was highest, even though males in these nests did not offer nearly as much care as their counterparts in higher nests.

As a further confirmation that the low intertidal zone is prime real estate when it comes to choosing a nest site, we found that the largest, most dominant males were much more likely to be found in low intertidal nesting sites.

For the plainfin midshipman, there seems to be a clear choice of nest site that benefits both the parent and their offspring. However, when there are few suitable nesting rocks in the low intertidal zone, males can be forced to nest either higher up in the intertidal zone, where it can get very hot, or in the subtidal zone, where it is always cold. Fathers that remain in the subtidal zone to nest do not need to provide as much care to their eggs. However, these males tend to attract fewer females and their eggs develop at a much slower rate. Those who instead move up high, make the best of a bad situation by providing more parental care to meet the higher demands of their offspring.

The intertidal zone is an exciting, unique, but challenging environment to study. We can study a broad range of environmental conditions at just one site. With a year’s experience of free diving under our belts, we hope to further explore the intertidal zone and its inhabitants to learn more about raising young in one of the Earth’s most dynamic ecosystems.