Liver worm (Contracaecum osculatum) in Eastern Baltic cod: growth, status, effects and biological information – input to management (TESLO & TORVÆKST)

The present report describes results of the two EMFF projects "Seal-related liver worm in Eastern Baltic cod: Status, effects and biological input for management (TESLO)” and "Biological press factors limiting the growth of Eastern Baltic cod (TORVÆKST)”, funded by European Maritime and Fisheries Fund and the Danish Fisheries Agency.

Knowledge about how parasites influence the health status, growth potential and mortality of fish is extremely limited, likely because it is difficult to quantify and describe, and parasites are rarely monitored. Furthermore, fish in the wild are exposed to various factors (e.g. unfavorable temperature, salinity, oxygen or food conditions), and it is difficult to separate the effects of these factors from those of parasites. Many parasites also have complex life cycles including several hosts, making their infection dynamics difficult to predict.

Grey seal is final host to the parasitic nematode Contraceacum osculatum (common name cods liver worm), and concurrent with the recovery of grey seal in the Baltic Sea, high infection levels with this parasite have been observed in cod in the central and southern parts of the Baltic Sea. Cod is one of several transport hosts to this parasite.

Field investigations have shown that infection intensity with liver worm in Eastern Baltic cod coincides with poor nutritional status. As the common name cod’s liver worm imdicates, this parasite sits explicitly in the liver of the cod. The liver is responsible for many vital processes including nutrient assimilation, bile production, maintenance of metabolic homeostasis and protein synthesis and also serves as an energy reserve and breeding capital for the fish. It is thus intuitive to think that a high liver parasite burden leads to reduced function of the organ with negative effects on the nutritional condition of the infected individual. It however remains unclear to what extent cod that are already thin due to food limitations are more susceptible to becoming infected, or whether the nutritional condition of the fish deteriorates when they have many of these liver parasites. This is especially relevant for Eastern Baltic cod, which has experienced food limitation for decades, and for several years has been a stock in distress. Disentangling potential effects of parasites on their hosts from effects arising in the wake of unfavorable abiotic and food conditions thus demands an interdisciplinary approach combining field and laboratory studies, and bioenergetic modelling.

The projects reveal that cod with many nematodes in their liver generally have a reduced energy status, both in terms of the amount of protein in the muscles and in terms of fat in the liver. These thin cod are possibly more susceptible to liver worms. However, the changed ratio between protein and glycogen energy in the fish found here points to a direct effect of the liver worm on the fish, and not an effect of being starved. In support, we also found that highly infected cod show signs of suffering from a severe liver disease reflected by changes in the blood plasma composition such as decreased plasma albumin and increased globulin levels, resulting in reduced albumin to globulin ratio, as seen in humans with chronic liver diseases. Histopatology revealed that cod with high infection loads of liver worms have pronounced inflammatory reactions and bleedings in their livers, pointing to reduced liver functionality. The growth experiment revealed that even when highly infected cod had access to ample food, they were not able to overcome the negative effects of high liver worm load on their nutritional condition.

Based on an extensive pan-Baltic analysis of liver worm in sprat, a main prey for cod, we show that infection levels of sprat with this parasite is in general low, although with some indications that sprat living adjacent to grey seal colonies have slightly higher infection levels. Yet, the results suggest that cod gets infected by addition prey items besides sprat. Building on these results, and energy content analysis of sprat throughout the year, we parameterized an individual-based bioenergetic model of growth in cod and show that independent of the starting size of the cod, the growth rate and condition factor decreased when the infection rate with liver worm increased. In addition, the model showed that heavily infected cod reached a 'point of no return' where their energy intake was so low that they could no longer get the energy required to cover the basal metabolism; after that the fish starved to death.

The spatial distribution of liver worm in cod will potentially vary over time, depending for example on how the number of grey seals develops in the future in the various areas. Detailed analysis of cod livers where the total number of liver worm in each organ is counted is however time-consuming and expensive, and we therefore developed the so-called ‘liver category method’ where individual livers are assigned a category between 0 and 4, depending on how many liver worms can be observed on the surface of the liver. We further developed area specific models which can be used to predict the total number of nematodes based on these categories. Based on this, and that results from the present projects combined clearly point to that infection load with liver worm is needed as an additional indicator of the health status of cod in the Baltic Sea, ICES made it a mandatory part of the routine sampling protocol on Baltic monitoring surveys from 2021 to assign a liver category to individual cod livers.