# Size Preference of Live Fish Prey in the Pellet-Consuming Pikeperch
## 1 Introduction
Environmental enrichment serves to reduce the selection effects of intensive rearing by providing a more complex environment during development. Its role is complex, as it not only improves fish welfare under intensive conditions but also allows individuals to perform better under natural conditions. Of the five categories of environmental enrichment, structural enrichment has been the main focus in recent years [18]. However, the other four strategies (sensory, occupational, social, and dietary enrichment) are also worthy of attention from a fish farming perspective. In the case of pikeperch, there are publications addressing animal welfare (structural enrichment [19], occupational [20], social [19,21], and dietary enrichment [22]), however, the consequence of dietary enrichment is not yet widely studied. The most important skills for captive-reared individuals during release into the wild are predator avoidance, foraging, and reproductive behavior. In the natural environment, for carnivorous fish, capturing live prey and learning tactics are of primary importance, which is facilitated by foraging training, mostly using direct exposure techniques [5]. Although there are publications available on the predation behavior of the pikeperch [23,24,25,26,27] information on the foraging of live prey by intensively farmed individuals is still limited. Foraging training studies in other species report variable successes, yet mostly result in improvements in the foraging behavior of naive fish. Studies generally aim to establish learning curves in order to determine the number of attempts sufficient to achieve an already acceptable post-release performance [5]. The training induces learning either through direct exposure to live prey or through social learning [13]. Increased foraging success has been found for Atlantic salmon (_Salmo salar_) [28,29], largemouth bass (_Microperus salmoides_) [30], jade perch, (_Scortum barcoo_) [31], turbot (_Scopththalmus maximus_) [32], and European grayling (_Thymalus thymallus_) [33]. In _esocids_ (muskellunge _Eosx masquinongy_ and tiger muskellunge (muskellunge \(\times\) northern pike _E. lucius_)), pre-release feeding experience did not significantly alter post-release prey consumption or growth, although the survival of naive (pellet-consuming) individuals was poorer [34]. In the case of the valley, under laboratory conditions, the experienced group consumed more prey compared to the naive group, but this difference could not be confirmed in pond experiments [35]. Changes in food preference have been observed in juvenile Atlantic salmon as a result of foraging training compared to the initial naive state [36]. The pikeperch, similar to its North American relative the walleye, could show an improvement in the success rate of the behavior based on practice [27; 35]. Exercise has probably a significant role in the kinematics of the behavior [37; 38]. The size of the prey can affect the learning process, larger prey can quickly lose (energetic) value due to higher handling costs and possibly lower predation success [39]. In wild-caught pikeperch, the preferred PPR value was 0.23 on average, while a maximum PPR of 0.63 was measured for 98 mm-TL individuals [40]. Based on a laboratory prey selection study [24], active prey choice can be observed in the species with a preference of 0.25 PPR. The aim of our study was to investigate whether and how intensively reared pikeperch show a prey size preference when foraging on live fish. In addition, we investigated how different prey sizes affect the prey capture process (number of attempts, predation latency) and whether the size of the prey ultimately captured is related to the prey size observed during attempts and predation success.