Too Many Fish on the Sea Floor

When fish like this sand goby aggregate, the density of their nests can often have a big impact on their success
When fish like this goby aggregate, the density of their nests can often have a big impact on their success (Image Credit: Laszlo Ilyes, CC BY 2.0, Image Cropped)
Spatial and temporal patterns of nest distribution influence sexual selection in a marine fish (2018) Wong et al., Oikos, doi: 10.1111/oik.05058

The Crux

When we monitor the fluctuations of a population, we often look at vital rates, a huge part of which is reproductive success. The success that males have in siring offspring can be hugely influenced by the density of a population, particularly when it comes to a breeding ground.

Larger males will often outcompete smaller males on such grounds, however in many species these males will often reach reproductive limits, at which point smaller males can benefit. Smaller males may also fare better in less dense populations, where females lack other individuals to compare them to. Our study today looks at variations in reproductive success of a nest-breeding fish species over two levels of density.

What They Did

The researchers created 6 different types of nesting sites, by nailing down tiles into the sea floor. Half of the nesting sites were clustered together, the others more spread out. The sites were also monitored at different times, to see, among other things, if larger males could sustain any advantage that they may have had over time. A third were collected after one day, another third after two days, and the rest were collected after five.

Success was judged by area of each plate covered by eggs, as well as a metric called “opportunity for selection” (basically how likely a fish was to get some). Fish were measured and collected to check size.

What They Found Out

Naturally, total eggs accrued increased over time. There were more eggs initially in the sparser areas, perhaps a result of less immediate competition benefiting the fish. However as time went on, the egg mass at dense sites became equal to that of the sparser sites.

Larger males also did very well initially in the more dense areas, however their advantage did indeed wear off after five days. There didn’t seem to be a selection for larger fish in the sparse areas.

Did You Know: Field vs. Lab

Often good science is easier to do in a controlled, laboratory setting. It’s easy to monitor your environmental variables, and easier to gauge your results. Depending on your necessary equipment, it can often be much cheaper. However often it’s difficult to gauge whether or not the results gained will be applicable in the field.

The problem with taking studies like this out into the field is that things get very complicated very quickly. What’s your intended site? Is it easily accessible? How much disturbance can you cause there? Can you monitor the impact of other species?

It’s somewhat of a dichotomy, and one all scientists struggle with.

Often, more desirable members of a species will fare better when they are around others, as this gives females something to compare them to

Often, more desirable members of a species will fare better when they are around others, as this gives females something to compare them to (Image Credit: Patte David, USFWS, CC0)


There are a lot of ambiguities in the results. The time scale (1, 2 and 5 days) isn’t ideal, though this is a typical situation where an experiment is resource-limited. This is typically an experiment which would take place in a lab, and I can only commend the researchers for pulling it off in the field.

So What?

This study infers some cool stuff about the relationship between nesting density and reproductive success. We can see that larger males generally have more of an advantage when they can easily be compared to smaller fish, and that they are likely to only retain their advantage for a short time when nesting starts. It also suggests that direct interference in courtship (which has been observed in other species and is naturally more likely in denser populations) may well reduce a population’s overall reproductive success.

Knowledge of these sort of vital rates are a great help in forecasting population fluctuations in species. They also give us a greater understanding of density dynamics in general, in particular when trying to resuscitate populations of declining species.

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