1. As species struggle to cope with rising ocean temperatures, temperate marine assemblages are facing major reorganisation. Many benthic species have a brief but critical period dispersing through the plankton, when they are particularly susceptible to variations in temperature. Impacts of rising temperatures can thus ripple through the population with community-wide consequences. However, responses are highly species-specific, making it difficult to discern assemblage-wide patterns in the life histories of different fish species.

2. Here we evaluate the responses to temperature in the early life histories of several fish species using otolith reconstructive techniques. We also assess the consequences of future warming scenarios to this assemblage.

3. We sampled recent settlers of 9 common species across a temperature gradient in the Mediterranean Sea and obtained environmental data for each individual. Using otolith microstructure we measured early life traits including pelagic larval duration (PLD), growth rate, settlement size, hatching and settlement dates. We used a GLM framework to examine how environmental variables influenced early life history parameters.

4. We show that increasing temperature results in considerable reduction in the dispersal potential of temperate fish. We find a nearly universal, assemblage-wide decline in pelagic larval duration (PLD) of between 10-25%. This was because, with increasing temperature, larvae grew quicker to their settlement size. Settlement size itself was less affected by temperature and appears to be an ontogenetically fixed process.

5. Given current estimates of ocean warming, there could be an assemblage-wide reduction in larval dispersal of up to 50 km across the Mediterranean, reducing connectivity and potentially isolating populations as waters warm. 06-Jan-2021