Using alternative biological information in stock assessment: condition-corrected natural mortality of Eastern Baltic cod

The Eastern Baltic cod abundance started rapidly to increase in the mid-2000s as evidenced by analytical stock assessments, due to increased recruitment and declining fishing mortality. Since 2014, the analytical stock assessment is not available, leaving the present stock status unclear and casting doubts about the magnitude of the recent increase in recruitment. Earlier studies identified main factors impacting on cod reproductive success to be related to the loss of two out of three spawning areas in the 1980s caused by lack of major Baltic inflows with a concurrent reduction in salinity and oxygen. Other important factors include prey availability for first-feeding larvae, egg predation by sprat and herring and cannibalism on juveniles, all in one way or the other related to the prevailing hydrographic conditions. These factors cannot explain increased reproductive success in the last decade, as the period was characterized by an absence of large-scale Baltic inflows since 2003 and persistent anoxic conditions in the bottom water of the deep Baltic basins. This questions the perception of the increased recruitment in later years and challenges our present understanding of cod recruitment dynamics in the Baltic Sea. In this contribution, we review evidence from the recent literature supplemented by information from latest research cruises to elucidate whether cod reproductive success indeed has increased during the last decade, and we suggest the key processes responsible for the recent dynamics in cod recruitment and outline directions for future research.

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Challenging ICES age estimation protocols: lessons learned from the eastern Baltic cod stock

ICES Journal of Marine Science ◽

10.1093/icesjms/fsw107 ◽

2016 ◽

Vol 73 (9) ◽

pp. 2138-2149 ◽

Cited By ~ 21

Author(s):

Karin Hüssy ◽

Krzysztof Radtke ◽

Maris Plikshs ◽

Rainer Oeberst ◽

Tatjana Baranova ◽

...

Keyword(s):

Quality Control ◽

Age Estimation ◽

Stock Assessment ◽

Alternative Methods ◽

Age Validation ◽

Error Matrix ◽

Visual Contrast ◽

Wide Range ◽

Eastern Baltic ◽

Baltic Cod

Abstract Over the recent decades, the International Council for the Exploration of the Sea (ICES) has set guidelines for best practise quality control of age estimation procedures. The applicability of these guidelines is assessed by reviewing the ageing issues of eastern Baltic cod (EBC) as a case study. Since the implementation of an age-based assessment of EBC in the beginning of the 1970s, the assessment has been hampered by the quality of the age composition data, in recent years to a degree that age-based assessment is no longer used. The reason for the age reading problems is the low visual contrast between growth zones in the otoliths which seems to be the result of complex interactions of the hydrography in the Baltic Sea with the cod’s biology and behaviour. Over the last 40 years, various expert groups have struggled to document and improve the agreement of age estimation between national otolith readers, standardize methods and age estimations through repeated exchanges and reference collections as well as an internationally agreed manual. Despite these initiatives the precision of the age estimations based on traditional ageing did not improve, with significant bias persisting between and within readers. Additionally, a wide range of alternative methods for deriving the age information necessary for stock assessment and for validation of the true age have been tested. However, these methods did not produce unbiased age estimates over the entire size and age range of the EBC stock. An age-validation is urgently needed. Deviations from the ICES guidelines identified are as follows: (i) the lack of rigorous quality control, particularly the auditing of national trends in age precision over the years using a reference collection and (ii) the implementation of an age error matrix in the stock assessment.

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Implications of stock recovery for a neighbouring management unit: experience from the Baltic cod

ICES Journal of Marine Science ◽

10.1093/icesjms/fsu060 ◽

2014 ◽

Vol 71 (6) ◽

pp. 1458-1466 ◽

Cited By ~ 21

Author(s):

Margit Eero ◽

Jakob Hemmer-Hansen ◽

Karin Hüssy

Keyword(s):

Stock Assessment ◽

Specific Area ◽

Management Unit ◽

Spatially Resolved ◽

Spatial Differences ◽

Long Time ◽

The Baltic ◽

Two Populations ◽

Eastern Baltic ◽

Baltic Cod

Cod in the Baltic Sea is assessed and managed as two separate stocks, i.e. eastern and western Baltic cod. The eastern Baltic cod has recently started to recover after several decades of severe depletion. In the present study, we suggest that the recovery of the eastern Baltic cod population has also substantially increased cod abundance in a specific area of the adjacent western Baltic management unit. This is investigated through long time-series of spatially resolved stock assessment data supplemented by genetic analyses of origin of the cod currently found in the transition area between the two populations. Due to immigrating cod from the east, there are currently large spatial differences in cod abundance and mean weight in the western Baltic management unit that raise new management concerns. First, the high abundance of cod of eastern origin found in the western Baltic management unit can mask the relatively poor state of the western Baltic cod population. Second, the current fishing mortality estimates for the entire western Baltic management unit, used as basis for fisheries management, are difficult to interpret as these are highly influenced by mixing of biological populations and the spatial distribution of fisheries.

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Physiological condition of Eastern Baltic cod, Gadus morhua, infected with the parasitic nematode Contracaecum osculatum

Conservation Physiology ◽

10.1093/conphys/coaa093 ◽

2020 ◽

Vol 8 (1) ◽

Author(s):

Marie Plambech Ryberg ◽

Peter V Skov ◽

Niccolò Vendramin ◽

Kurt Buchmann ◽

Anders Nielsen ◽

...

Keyword(s):

Gadus Morhua ◽

Physiological Condition ◽

Parasitic Nematode ◽

Nutritional Condition ◽

Natural Mortality ◽

Risk Of Predation ◽

High Infection ◽

Contracaecum Osculatum ◽

Eastern Baltic ◽

Baltic Cod

Abstract Establishing relationships between parasite infection and physiological condition of the host can be difficult and therefore are often neglected when describing factors causing population declines. Using the parasite–host system between the parasitic nematode Contracaecum osculatum and the Eastern Baltic cod Gadus morhua, we here shed new light on how parasite load may relate to the physiological condition of a transport host. The Eastern Baltic cod is in distress, with declining nutritional conditions, disappearance of the larger fish, high natural mortality and no signs of recovery of the population. During the latest decade, high infection levels with C. osculatum have been observed in fish in the central and southern parts of the Baltic Sea. We investigated the aerobic performance, nutritional condition, organ masses, and plasma and proximate body composition of wild naturally infected G. morhua in relation to infection density with C. osculatum. Fish with high infection densities of C. osculatum had (i) decreased nutritional condition, (ii) depressed energy turnover as evidenced by reduced standard metabolic rate, (iii) reduction in the digestive organ masses, and alongside (iv) changes in the plasma, body and liver composition, and fish energy source. The significantly reduced albumin to globulin ratio in highly infected G. morhua suggests that the fish suffer from a chronic liver disease. Furthermore, fish with high infection loads had the lowest Fulton’s condition factor. Yet, it remains unknown whether our results steam from a direct effect of C. osculatum, or because G. morhua in an already compromised nutritional state are more susceptible towards the parasite. Nevertheless, impairment of the physiological condition can lead to reduced swimming performance, compromising foraging success while augmenting the risk of predation, potentially leading to an increase in the natural mortality of the host. We hence argue that fish–parasite interactions must not be neglected when implementing and refining strategies to rebuild deteriorating populations.

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Effects of varying natural mortality and selectivity on the assessment of eastern Baltic cod (Gadus morhua Linnaeus, 1758) stock

Journal of Applied Ichthyology ◽

10.1111/jai.13202 ◽

2016 ◽

Vol 32 (6) ◽

pp. 1032-1040 ◽

Cited By ~ 1

Author(s):

J. Horbowy

Keyword(s):

Gadus Morhua ◽

Natural Mortality ◽

Eastern Baltic ◽

Baltic Cod

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It’s elemental, my dear Watson: validating seasonal patterns in otolith chemical chronologies

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/cjfas-2020-0388 ◽

2020 ◽

Author(s):

Karin Hüssy ◽

Maria Krüger-Johnsen ◽

Tonny Bernt Thomsen ◽

Benjamin Dominguez Heredia ◽

Tomas Naeraa ◽

...

Keyword(s):

Age Estimation ◽

Gadus Morhua ◽

Stock Assessment ◽

Early Spring ◽

Fish Stock ◽

Late Winter ◽

Seasonal Patterns ◽

Traditional Age ◽

Eastern Baltic ◽

Baltic Cod

Accurate age data is essential for reliable fish stock assessment. Yet many stocks suffer from inconsistencies in age interpretation. A new approach to obtain age makes use of the chemical composition of otoliths. This study validates the periodicity of recurrent patterns in 25Mg, 31P, 34K, 55Mn, 63Cu, 64Zn, 66Zn, 85Rb, 88Sr, 138Ba, and 208Pb in Baltic cod (Gadus morhua) otoliths from tag-recapture and known-age samples. Otolith P concentrations showed the highest consistency in seasonality over the years, with minima co-occurring with otolith winter zones in the known-age otoliths and in late winter/early spring when water temperatures are coldest in tagged cod . The timing of minima differs between stocks, occurring around February in western Baltic cod and one month later in eastern Baltic cod; seasonal maxima are also stock-specific, occurring in August and October, respectively. The amplitude in P is larger in faster-growing western compared to eastern Baltic cod. Seasonal patterns with minima in winter/late spring were also evident in Mg and Mn, but less consistent over time and fish size than P. Chronological patterns in P, and to a lesser extent Mg and Mn, may have the potential to supplement traditional age estimation or to guide the visual identification of translucent and opaque otolith patterns used in traditional age estimation

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