Effect of Marine Hypoxia on Baltic Sea Cod Gadus morhua: Evidence From Otolith Chemical Proxies

The Baltic Sea contains the world’s largest anthropogenic deoxygenated zone, with increasing episodes and areal extent of hypoxia/anoxia. Atlantic cod in the Baltic has suffered a loss in condition which has been attributed mainly to hypoxia. Otoliths, the aragonitic structures that form part of the hearing/balance system in fishes, accumulate Mn in the presence of hypoxia and other reducing environments. Otoliths grow over the lifetime of fishes, and thus life-long records of hypoxia exposure exist for each individual fish. However, otolith Mn/Ca ratios are also sensitive to growth effects. We tested a new proxy to at least partially account for growth: Mn/Mg, since Mg levels reflect metabolic activity but not hypoxia. This and other elemental proxies were parsed annually from the otoliths to reconstruct lifetime histories of mean, maximum, and cumulative values of this proxy as well as others (Sr/Ca) that inform us about salinity conditions. We analyzed cod from five different time periods: Neolithic (4500 YBP, a normoxic baseline), 1980s, 1990s, 2000s, and 2010s – under different hypoxia intensities, assessing fish growth and condition in relation to hypoxia experience recorded by otolith proxies. Fish growth decreased with increasing hypoxia exposure; condition at capture (measured by Fulton’s K index) showed a strongly positive relation to growth indexed by magnesium (Mg/Ca). We conclude that cod otolith chemistry proxies not only inform about the hypoxia, growth, and metabolic status of cod, retrospectively throughout life, but also reflect the worsening situation for cod in the Baltic

Discrimination potential of otolith chemistry to distinguish two parapatric species of flounder (Platichthys) in the Baltic Sea

Baltic Sea flounder were recently split into two species, the offshore spawner Platichthys flesus and coastal spawner Platichthys solemdali. The two species can only be distinguished based on egg and sperm characteristics and via genetic analyses, which limits the species identification methods of larvae and juveniles to molecular techniques. We investigated whether otolith chemistry could be used as an additional tool to identify flounder to species level. We tested for species-specific differences in otolith multi-elemental signatures and spatial consistency of those differences for the early life stages of flounder in three areas of the central Baltic Sea (ICES SD 24-28), where the distribution of both species overlaps. Otolith chemistry signatures obtained through maternal transfer (i.e. core chemistry) and signatures that reflect the post-hatching phase were not significantly different between species. Species-specific differences at the sub-regional scale were only found for the Latvian coastal survey area for multiple elements (Ba, Cu, Mg, Pb, Sr and Zn), but were insufficiently distinct for reliable species discrimination. Geographic classification of age-0 juveniles to survey area was more successful than classification to species, which was reflected by a spatial trend in otolith Sr:Ca that followed the salinity gradient and higher Mn:Ca and I:Ca for Latvian individuals. Otolith chemistry of early life flounder from the Baltic Sea reflects spatial variability in environmental conditions but does not differentiate between the two flounder species in sympatric habitats

Tracking Fish Lifetime Exposure to Mercury Using Eye Lenses

Mercury (Hg) uptake in fish is affected by diet, growth, and environmental factors such as primary productivity or oxygen regimes. Traditionally, fish Hg exposure is assessed using muscle tissue or whole fish, reflecting both loss and uptake processes that result in Hg bioaccumulation over entire lifetimes. Tracking changes in Hg exposure of an individual fish chronologically throughout its lifetime can provide novel insights into the processes that affect Hg bioaccumulation. Here we use eye lenses to determine Hg uptake at an annual scale for individual fish. We assess the widely distributed benthic round goby (Neogobius melanostomus) from the Baltic Sea, Lake Erie, and the St. Lawrence River. We aged layers of the eye lens using proportional relationships between otolith length at age and eye lens radius for each individual fish. Mercury concentrations were quantified using laser ablation inductively coupled plasma mass spectrometry. The eye lens Hg content revealed that Hg exposure increased with age in Lake Erie and the Baltic Sea but decreased with age in the St. Lawrence River, a trend not detected using muscle tissues. This novel methodology for measuring Hg concentration over time with eye lens chronology holds promise for quantifying how global change processes like increasing hypoxia affect the exposure of fish to Hg

PII: S0921-8009(99)00008-7

EDITORIAL The ecology of ecosystem services: introduction to the special issue Throughout history, humankind has enjoyed a love-hate relationship with Nature, praising its bounty, fearing catastrophe, or challenging and conquering wilderness and sea. Regardless of our sense of distance from Nature, humans are nonetheless one species out of millions of others on Earth, one with an exceptional ability to harness a vast spectrum of energy sources, materials, and organisms for our welfare. As we exit the second millennium, we enter a world in which our impacts on the environment no longer can be ignored on global scales. In the coming century, our species, numbering roughly 10 -12 billion, will be squeezing many natural resources to and in excess of their limits. We will also continue to affect profoundly biogeochemical and hydrological processes that occur at scales ranging from microbial to global-atmospheric. How did we get here? By doing what all organisms do: we use resources to survive and we reproduce successfully. As highly social creatures, we have been effective at organizing and developing infrastructure and mores that sequester resources and protect us from the environmental adversities of weather, disease, starvation, etc. The development of civilization and culture has blinded many to the fact that humans are irrevocably tied to the natural world, a blindness exacerbated during the fossil-fuel era. Many societies have become philosophically and mentally 'disembedded' from the biophysical milieu (see Borgströ m-Hansson and Wackernagel, this issue), despite the fact that socio-economic development ultimately depends on the dynamic capacity of ecosystems to support it. Although ecologists and other environmental scientists have long understood the strong coupling between humans and the rest of Nature, many choose to ignore this relationship and instead derive knowledge about the natural world by studying 'pristine' situations. Today, increasing numbers of these scientists are re-examining the Man-Nature links and attempting to make these clear to the public as well as to their colleagues. For example, Wilson (1992) drew attention to the importance of biodiversity, and to the emerging crisis of massive species extinction due to human alterations of ecosystems. Today, few people question the human dominance of the plane

Diverse migration tactics of fishes within the large tropical Mekong River system

Fish often migrate to feed, reproduce and seek refuge from predators and prevailing environmental conditions. As a result, migration tactics often vary among species based on a diversity of life history needs, although variation within species is increasingly being recognised as important to population resilience. In this study, within- and among-species diversity in life history migratory tactics of six Mekong fish genera was examined using otolith microchemistry to explore diadromous and potamodromous traits. Two species were catadromous and one species was an estuarine resident, while the remaining three species were facultative in their migration strategies, with up to four tactics within a single species. Migrant and resident contingents co-existed within the same species. Management, conservation and mitigation strategies that maintain connectivity in large tropical rivers, such as effective fishway design, should consider a diversity of migration tactics at the individual level for improved outcomes

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring.[GRAPHICS]

Reading the biomineralized book of life: expanding otolith biogeochemical research and applications for fisheries and ecosystem-based management

AbstractChemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring. Graphical abstract</jats:p