Close to home: what drives the distribution of intertidal rockpool fishes?

Read below for a great summary by Kai Paijmans on his research examining the distribution of rockpool fishes. Kai’s project was for a 3rd year UOW research subject, where he was supervised by Dr. Marian Wong and Ben Gooden.

Close to home: what drives the distribution of intertidal rockpool fishes?

Rocky intertidal shores are right on the edge of our home – between the known of the shores and the vast unknown of the world’s oceans. Although regularly overstepped by many of us, rockpool life is largely overlooked in favor of large charismatic and untouchable creatures of the open and comparatively inaccessible ocean. It is important to understand the ecological processes occurring on rocky shores as they are right on our doorstep; heavily used for, and potentially impacted upon by recreation.

Rockpools contain fascinating communities of weird creatures that many of us peer at when we walk along the shore. I have studied rockpool fishes on shores surrounding Wollongong in South Eastern Australia. My study aimed to identify what factors drive the distribution of rockpool fishes across intertidal rocky shores. I used a bilge pump to empty pools so as all the fish within a given pool could be captured, identified and measured before release.

A rockpool at Sandon Point (NSW Australia) is emptied prior to fish collection, identification and measurement.

A rockpool at Sandon Point (NSW Australia) is emptied prior to fish collection, identification and measurement.

Due to the periodic isolation of rockpools during low tide these ‘micro habitats’ produce dynamic and harsh environmental conditions. Large variation in temperature, dissolved oxygen and pH mean that the organisms that inhabit pools need to be tolerant to a dynamic environment and actively select pools with favorable environmental conditions.

If fish species base their pool selection solely on favorable environmental conditions, we could expect that the distribution of different fish species would be correlated with certain environmental conditions. This has been found to be the case with several rockpool fish communities studied globally.

Interestingly my studies found that the distribution of fishes in rockpools in South Eastern Australia was correlated not with abiotic (non-living) environmental conditions, but rather with biotic factors: Namely the presence of the Cocos Frill Goby (Bathygobius cocosensis). I found that the abundance of other fish species was significantly less in pools where the Cocos Frill Goby was present.

The Cocos Frill Goby (Bathygobius cocosensis) is likely to be competitively dominant on South Eastern Australian rocky shores, limiting the distribution of other fish species.

The Cocos Frill Goby (Bathygobius cocosensis) is likely to be competitively dominant on South Eastern Australian rocky shores, limiting the distribution of other fish species.

These results imply that the Cocos Frill Goby is competitively dominant within this ecosystem and therefore limits the distribution of other fish species, although the precise mechanism by which the Cocos Frill Goby excludes other species remains to be determined. The findings of this study provide the first evidence that competition may be a key determinant of the distribution of fishes on the intertidal rocky shores of South Eastern Australia, challenging the notion that abiotic environmental parameters are the main driving factors of fish distributions on intertidal rocky shores.

My future studies may confirm that competition among fish species is a key factor driving the large diversity of species that live in rockpools on the intertidal rocky shore.

Any questions, thoughts, feedback or constructive criticisms are welcome.

Kai Paijmans

7 thoughts on “Close to home: what drives the distribution of intertidal rockpool fishes?

  1. Margie

    Hi Kai,

    Interesting read. I’m am somewhat obsessed with rock pools and their inhabitants, I’m not a scientist, just an ocean lover. I had an aquarium running for a year and a half that contained only organisms found in the rockpools down the end of my street at Headlands. The variety of stuff we found and had in the tank was mind boggling and it seemed that the more you look, the more you find. As soon as I thought I’d seen everything, something new would be found!

    Anyway, we had one of these Cocos Frill Gobies in our tank for a while and I thought I could maybe lend some insight to this part of your conclusion:
    ‘the precise mechanism by which the Cocos Frill Goby excludes other species remains to be determined.’
    From my experience of observing my tank, I can say that the precise mechanism that the Cocos Frill Goby uses excludes other species is that it eats them all! When I had the wee fish in my tank, I had no idea what it was exactly, just an idea that it was some kind of goby. It almost instantly became known as the ‘bully’ of the tank, chasing the others around and eventually fish (and shrimp) started disappearing. A few times I would check on the tank in the morning and I could see their tails or antennae still sticking out of its mouth!
    I ended up re-releasing it. It definitely seemed liked some kind of poetic justice when within seconds of release, it was picked off by a passing seagull 🙂

    Maybe you already suspected that they ate everything but I thought I’d share my observations just in case 🙂


    P.s. Did you have many ‘casualties’ using the bilge pump?


  2. Kai Paijmans

    Hi Margie,

    First of all thanks for the interest. I have heard several reports of the Cocos Frill Goby being aggressive towards and predatory of other fishes (and now shrimp as well!). All going to plan I will further my studies of rockpool fishes as part of my honors year next year. It will be interesting to see the extent to which other species are excluded from pools, and the size of fishes impacted by the “potentially” (ie. yet to be confirmed) aggressive behavior of the Cocos frill goby.

    There was no “casualties” using the bilge pump. It draws water quite slowly and has a cage around the water intake to avoid the sucking up of fishies 🙂




  3. Richard

    Interesting read! I’m totally at sea (pun intended) in this area, having only studied marine biology and benthic marine paleoecology many, many years ago at University of Santa Barbara. So I realize my comments may be uninformed. If so, please accept my apology in advance.

    Caveat aside, the question that immediately arose in my mind after reading the initial post was, “How, exactly, does a particular fish get into a particular tide pool?”

    Seems like there might be several options (not meant to be mutually exclusive):
    (1) Fish is (are) captured by some random wave that tosses it (them) high into a pool that’s already above the current tidal level. Once in the pool there’s nowhere to go, and the fish stays there (unless eaten or vacuumed up) until the tide comes back in. (Possible sub question: Does the distribution of fish in tidal pools match or differ from the ordinary geospatial distribution in the ocean at high tide, except perhaps for the impact of the Cocos Frill Goby?)
    (2) Fish is (are) lurking along the bottom of the ocean and basically don’t notice the tide going out. They’re just stranded and probably are unaware that they have been cut off from the rest of the ocean. (Possible sub question: Does the same individual fish get trapped in the same pool more than once? Tagging the fish and following the distribution over time might be very revealing.)
    (3) Fish is (are) engaged in some activity (e.g., feeding or fleeing from a predator) which has a survival factor that involves staying into a local depression in the sea floor — even when the tidal currents might clearly indicate the tide is going out.


  4. kaipaijmans

    Hey Richard,

    thanks for the read. This is great stuff, you have got me thinking!

    Due to the patterns of distribution identified in my earlier studies, it is evident that there is some form of organisation of fish communities within pools. I suspect that the pools serve as “microhabitats” and that the community of fish within a given pool is organised and stable through time (rather than random). Although as you have highlighted, inundation and connectivity of pools during high tide is likely to facilitate movement of fish (for reasons such as predation pressure, feeding behaviour etc.).

    I am going to be tagging a whole lot of fish early next year with the aim of identifying weather or not fish communities within pools are temporally stable. Further, I am interested in understanding what factors drive the community compositions of fish and importantly by what mechanisms community organisation occurs.

    I suspect that intraspecific competition (between the Cocos Frill Goby and other species) is a key factor driving distribution patterns. As part of further studies I will also be doing a bunch of contest experiments in order to understand if biotic factors (size of fish and sex of fish) effect contest outcomes and also wether abiotic factors (temperature, dissolved oxygen etc.) affect these competitive interactions.

    So in relation to your question: “How, exactly, does a particular fish get into a particular tide pool?”. Inundation and connectivity of pools at high tide allows fish movement. Due to patterns of fish distribution being present, I suspect that fish community organisation within these “microhabitats” is driven by competition. Hopefully my future studies will spread some light on the drivers and mechanisms of distributions.


    1. Penelope

      Hi Kai,
      I realise its been a while since you posted this but thought I’d give it a shot. I’m a phd student at macquarie uni and currently have 24 cocos in my lab. I’ve worked with them for a few years but only the ones around Dee Why and Chowder Bay. If you could shed some light on where you collected them from, I’d love to hear from you. Please email me at




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