Shelter Competition Between Mature Non-Indigenous Western Tubenose Goby ( Proterorhinus Semilunaris ) and Immature Invasive Round Goby ( Neogobius Melanostomus ) for Plants and Rocks

Non-indigenous western tubenose goby Proterorhinus semilunaris and round goby Neogobius melanostomus have rapidly increased in numbers and displaced native ichthyofauna in invaded ecosystems. This displacement has been predominantly linked to competition for resources and shelters. Similarly, P. semilunaris was displaced in many localities after introduction of N. melanostomus but the mechanisms of displacement are still not clear. Within the present study shelter competition laboratory studies were completed with P. semilunaris as resident fish before the introduction of either a sized matched P. semilunaris or N. melanostomus intruder. The fish competed for shelters which resembled a rock or plant. Time in shelter and accounts of direct aggression, indirect aggression and the guarding of shelter were also recorded for each fish. Shelter competitions showed P. semilunaris spent more time in plant shelters but N. melanostomus spent similar time periods in both shelter types. In intraspecific pairings, resident fish spent significantly more time in the shelters and exhibited significantly higher direct aggression and shelter guarding, especially for plant shelters. In interspecific pairings, there was no significant difference in the time spent in shelter by resident P. semilunaris and intruder N. melanostomus in either shelter type at day or night. However, in interspecific pairings, the intruder fish showed significantly higher levels of all aggression types. In intraspecific pairing, the concept of ‘resident wins’ was observed. However, in interspecific pairings, N. melanostomus was able to displace the resident fish. The higher aggression of N. melanostomus in shelter competition could account for greater invasive success and the reduction of P. semilunaris observed in the wild.