In fisheries characterised by multiple target species, and in particular those with joint harvesting technologies, managers can inadvertently impact numerous species under management by way of single species focused regulatory measures. By curtailing the capacity of fishers to attain revenues from a particular species, effort is displaced and often directed towards alternative species. Treating fishers as utility maximising agents, in which utility (and thus species targeting effort) is positively associated with expected revenue and negatively associated with revenue variability, this paper employs portfolio theory to analyse fishers’ species targeting choices in the Irish Hake-Monkfish-Megrim and Cod-Haddock-Whiting fisheries. The particular concern is adjustments to targeting decisions when species-specific quota constraints are implemented. The analysis uses the utility maximising assumption in a mean variance optimisation framework to approximate fishers’ objective function. Species targeting behavioural changes, identified as changes in the species composition of fishers’ optimal harvest portfolio, suggest significant displacement of fleet into alternative fisheries occurs when barriers to such alternation do not exist.