The view of a data scientist - BONUS BIO-C3 & BONUS INSPIRE
It’s not a time to be picky, and sprat knows it better
Sprat and herring eat the same food, but should they compete, then the sprat will more likely win. That is because sprat is less picky about what it eats, and also more successful in finding its meal than herring. And the competition is tough, since food is limiting for both species these days in the Baltic Sea.
These were very shortly the main findings of the BONUS INSPIRE clupeid feeding survey in the summer of 2015, where >1500 fish were caught and cut open, to have a close look at what they had eaten.
Herring and sprat are the two most abundant commercially exploited fish species in the Baltic Sea. They’re both smallish, look similar, eat likely the same food, and behave similarly – school together, and prey on pelagic zooplankton.
Main difference has lately been their stock size in the northern open Baltic Sea: since the mid-eighties, abundance of sprat has been high, while herring has been low (section 1.8.18 of latest ICES WGBFAS report).
The high abundance of sprat is explained mostly by the collapse of the cod, the main predator of sprat in the Baltic Sea, around 1980s. While the population of cod diminished, and retreated to southern Baltic Sea, sprat became increasingly abundant in northeastern territories (Baltic Proper, and entrance to Gulf of Finland).
The high abundance of a species can result in food competition between the individuals, and that was also happening for the sprat – in areas where their abundance increased, the body size and growth decreased (Casini et al. 2014). But the negative effect was not only limited to sprat itself, also the growth of herring – the main food competitor of sprat - was affected by the increasing sprat stock (Casini et al. 2010).
While the negative effect of sprat on herring body condition was established from negative correlation between the abundance of sprat and herring growth, the actual food competition still needed to be confirmed with actual data of what the two species eat in the same place and same time. Therefore, the herring and sprat feeding surveys were carried out in INSPIRE project.
What’s in the menu?
Tiny crustaceans – mainly copepods – were the most typical dish for both fish.
It also seems that the fish like some of the copepod species better than others. In particular, the Eurytemora affinis in Gulf of Finland, and Temora longicornis in open Baltic Sea were clearly sought for, as their share in stomachs was much higher than their share in the water.
The diet of sprat was little bit more diverse: the total list of species that were recovered from sprat stomachs was longer, but also the number of species that were found within one stomach was higher for sprat. Also the share of fish that had eaten something was higher for sprat – the starving herring were quite common in this survey.
At the same time, both species seem to be limited by the food, because their stomachs were fuller when zooplankton biomass in the sea was higher. And the diversity of zooplankton did no good for the fish: the more diverse the zooplankton, the more difficult it was for sprat and herring to get their meal – monocultures of their two favorite copepods would have been the best!
This kind of detailed studies of fish stomachs are time-consuming and tedious, but they're also the only way to gain direct evidence of the actual resources that different species rely on, or whether or not they compete with each other for the food, or are food limited in general.
Results of the herring and sprat feeding survey will soon appear as a paper by Ojaveer et al. in ICES JMS.