Foraging efficiency and capacity of non-native round goby (Neogobius melanostomus) under various biotic conditions

The worldwide trend of continually increasing records of non-native species has  resulted in an  increasing number of potentially invasive  species that have  a dramatic impact on  native biota. In the case of a predator introduction, its impact is mainly displayed via the consumption rate of native species. The non-native fish – round goby  (Neogobius melanostomus) poses a  serious threat to native macrozoobenthos, which  represents a  main  diet component in invaded or seasonally colonized regions. Therefore, a quantification of the consumption rate of invasive,  emergent, or potentially invasive  predators is a keystone step to their effective management and prevention of continuous spreading. The functional response (FR) evaluating predator-prey interaction based on predator foraging efficiency  has  become an effective tool in ecological modelling to determine a predator foraging efficiency.  Moreover, this  method allows   to  experimentally determine the  foraging efficiency  of  the invasive   species under various biotic  factors, as in the present Ph.D. thesis, or to compare the impact of non-native species with  its  native analog. In the present thesis, various prey  types in density gradients were offered to the round goby  to determine the FR and  impact potential (IP) of this  species. Based on the results, the foraging efficiency, foraging capacity in multiple prey systems as well as under stress caused by presence of apex  predator or regarding population size structure in the round goby  were quantified.
The round goby  showed a type II FR in all experiments, including different prey  types, such as isopods, or chironomid larvae  or early stages of crayfish.  Neither the body  size of the round goby  as a predator nor the tail-flip antipredator strategy of the crayfish  as a prey changed the type of FR. The round goby  showed equal handling time and  significantly lower  attack rate to isopods than crayfish  when both prey types were offered separately. The combination of both prey types led to prolonging the round goby  handling time and  increased or decreased attack rate depending on  the ratio of both preys. Despite that, no  prey  selectivity was  detected  in the round goby  except for  the highest prey  density (100 individuals.box-1), where crayfish were preferred. The round goby  can  seriously threaten  native endangered crayfish  species in smaller tributaries where it continually or seasonally spreads, regarding its  equal foraging towards tested prey  and  lower  reproduction rate of crayfishes.
Our  results  also   confirmed that  the  predator  (round goby) body   size   represents  an important parameter influencing its  foraging efficiency  towards isopods. Small  individuals showed significantly lower  attack rates and  longer handling times than large  ones. However, the lower  per capita foraging efficiency  of  the small  individuals is  compensated by  their numerical dominance in population structure. Therefore, the small-sized cohort of the round goby  reached the highest overall  impact potential (IP) compared to medium or  large-sized cohorts with  lower  proportional abundance in  the investigated population. Nevertheless, overlooking the body  size structure of the round goby population in calculating the species IP led to only a minor  overestimation of the IP in the investigated population. However, the body size cohort used to estimate the round goby  IP should always  be cautiously chosen reflecting body  size structure of particular investigated population to prevent misestimation.
Although the round goby  is a voracious predator of macrozoobenthos, our  results showed that chemical cues signalizing predation risk by the apex  predator (European eels) remarkably
influenced both attack rate and  handling time of  the round goby  as  a  mesopredator.  The chemical cues  of  the  European eel   combined  with   conspecific  alarm   cues  significantly decreased the attack rate of  the  round goby.   On  the  contrary, the  handling time was significantly prolonged if the round goby  was  exposed to chemical cues of European eel  fed by heterospecific prey.  These  findings suggest that optimized management of apex  predator populations might lead  to a decrease in the impact of the round goby  at the invaded areas driven  by its lowered consumption rate.
The round goby’s  non-selective foraging and effective consumption of a wide prey spectrum make this  fish a serious threat to native freshwater biota. Even small  round goby  individuals having   lower   consumption rate  may  reach  a  high   total  impact potential  upon invaded ecosystem regarding their abundance population. Despite that, it  seems that native apex predators  through their non-consumptive effect can  noticeably decrease the overall  round goby’s   negative impact. However, that  relies   on  rehabilitating the  native apex   predator population,  which   depends  on   restoring  freshwater  ecosystems  currently  facing   high
anthropogenic pressures.

This work  was  carried out with  the support of  the South Bohemian Research Center of Aquaculture and  Biodiversity of Hydrocenoses (CENAKVA, ID 90238) in accordance with  the objectives of the european consortium DANUBIUS-RI.