Frank Kirschbaum

Status and Management of Eurasian Sturgeon: An Overview

This paper is an attempt to delineate the causes for the dramatic decline of the most part of European sturgeon species and the recovery measures taken. Overfishing, damming, pollution, poaching, water pumping, and dredging are the main quoted causes for the decline of sturgeon populations. 2 to 6 causes are cited simultaneously. Ranking the causes is mostly impossible. Various measures were taken to regulate the fisheries; among them were size limits, restricted catching periods, quotas, no measures at all, and restricted catching areas which accounted for 50%, 50%, 21%, 21% and 14%, respectively. Efficiency of police control, fines, fish-passes, and stocking are reported. Some explanation to apparent contradictions between biological knowledge and sturgeon status are given. There are suggestions in: trade control, ex situ conservation, fisheries’ biology and/or population management, and the increasing relative weight of sturgeon fisheries.

Reproductive strategies and developmental aspects in mormyrid and gymnotiform fishes

Comparative data on the reproduction in captivity and ontogenetic development of six of mormyrid species and seven gymnotiform species are reported. Mormyrid fishes: egg diameter ranged from 1.8 (Petroephalus soudanensis) to 3.0 mm (Hippopotamyrus pictus. Campylomormyrus phantasticus); fecundity (egg number per spawning) from 28 to 215 (Pollimyrus isidori) and 121 to 1662 (Campylomormyrus cassaicus); spawning intervals from 5 to 20 days (Pollimyrus isidori) and 15 to 80 days (Campylomormyrus cassaicus). Pollimyrus isidori is the only species exhibiting parental care (in the male sex). Gymnotiform species: egg diameter ranged from 1.7 (Brachyhypopomus pinnicaudatus) to 3.0 mm (Sternopygus macrurus, Rhamphichthys sp., Gymnotus carapo); fecundity (egg number per spawning) from 1 to 105 (Apteronotus leptorhynchus) and from approximately 500 to approximately 1000 (Rhamphichthys sp.); spawning intervals from 2 to 5 days (Eigenmannia lineata) and from 20 to 41 days (Rhamphichthys sp.). Sternopyguls macrurus exhibited parental care (in the male sex) by guarding the eggs, whereas Gymnotus carapo revealed to be a mouthbreeder (guarding free embryos) in the male sex. Gonad maturation could be provoked by increase of conductivity alone in all mormyrid species tested and in several gymnotiform species. Four different stages of morphological development (hatchlings, larvae at beginning of exogenous feeding, juveniles, adults) are described in both taxa. The reproductive strategies of the gymnotiforms are considered more diverse than those of the mormyrids.

Acipenser sturio Recovery Research Actions in France

The European Atlantic sturgeon Acipenser sturio, formerly present throughout Europe, is currently represented by a very critically endangered population in the Gironde-Garonne-Dordogne basin in France. In spite of its protected status in France since 1982, the relict population has continued to decline. Better knowledge of its biology and ecology has been acquired with regard to potential spawning grounds, migrations both downstream into the estuary and on the continental shelf, strength of the last representative cohorts in the estuary, the inhabited area and feeding habits in the Gironde estuary, adaptation of stocked fish to the wild, and characteristics of wild brood fish (frequency, date, sex, reproductive status). Due to the dramatic decline in brood fish by-catch, only four artificial reproductions were successful between 1981 and 2006, the last taking place in 1995. This was the first opportunity to grow larvae and to mark fingerlings for stocking. From the early 1990s, brood-stock building appeared to be the only way to potentially produce fingerlings on a regular basis. As there were no references in either husbandry or related ecophysiology standards, a precautionary strategy was adopted. The most critical environmental factors are salinity, temperature, and light regime. Some encouraging results (semen with motile spermatozoa) were recorded from reconditioned adult males held in brackish water. Some females initiated only partial ovarian development and therefore we introduced some changes, especially in the temperature regime. Complementary investigation and data have been forthcoming through European cooperation on husbandry, feeding, genetics, and endocrinology and this will be developed. Further actions are briefly presented.

Control of gonadal maturation and regression by experimental variation of environmental factors in the mormyrid fish, Mormyrus rume proboscirostris

We used spawning, external appearance of the females and in part histology of gonads of the African mormyrid fish Mormyrus rume proboscirostris to evaluate the reaction of the fish to experimental variation of environmental factors. Conductivity changes are sufficient to induce gonadal maturation (decrease of conductivity) and regression (increase of conductivity), respectively. Increase of water level and simulation of rain at constant conductivity values did not induce gonadal maturation and spawning. Neither males nor females of M. rume proboscirostris showed postbreeding refractoriness. Social stress apparently prevented complete gonadal maturation and successful spawning. The females laid on average 364 eggs per fractional spawning; the spawning intervals were 21 days. Hatching rate amounted to about 40% in all experiments taken together.

Fatty Acid Composition in Sturgeon Caviar from Different Species: Comparing Wild and Farmed Origins

The fatty acid compositions of both lipid classes, triglycerides (TG) and phospholipids (PL), of caviar from 34 specimens of wild caught and 12 farmed specimens were measured. The investigated caviar samples of wild caught sturgeon comprised Huso huso, Acipenser gueldenstaedti, Acipenser stellantus and Polyodon spathula of different origins and those of farmed sturgeon from Acipenser baeri, A. stellatus, and P. spathula, Oleic acid (18 : 1 n - 9) was the dominant fatty acid in TG of all caviar samples. In comparison with the TG, the PL have an elevated content of polyunsaturated fatty acids such as arachidonic (20:4 n-6), eicosapenlaenoic (20 : 5 n-3) and docosahexaenoic acid (22:6 n - 3). In the TG were observed some differences in the fatty acid composition between the caviar samples of the various species, especially in 18 : I n-9 and 20.5 n - 3 hut not in the PL. The proportion of n - 3 to n - b polyunsaturated fatty acids (PUFAs) was independent from the fish species and origins and did not reveal significant differences. The amount of n - 3 PUFAs was in average three times higher compared to n - 6 PUFAs. However, the fatty acid composition of the caviar depends on the origin of the sample, probably caused by differences in the food web. The caviar of farmed sturgeon contained a significant higher amount of linoleic acid (18:2 n - 6) and a lower amount of arachidonic acid, when compared with caviar from wild sturgeon. An explanation could he that sturgeon are not able to convert 18 : 2 n - 6 into 20 : 4 n - 6.

Processing techniques for caviar and their effect on product composition

Various techniques are established to process sturgeon roe to caviar. These processing techniques have significant impact on product composition and quality and thereby marketability. Analysis of caviar samples revealed drastic differences with regard to the gross composition of ova, including water-, protein-, and fat content, which could be linked with the processing techniques utilised. The differences in the preparation processes of the different samples were indicated by the salt contents (NaCl), varying between 3.2% to almost 11.8% between the samples. The elevated salt contents result in dehydration, which increased the concentration of lipids and proteins in a linear pattern. In 28 samples included in the analysis the water contents ranged from 31.5% wet weight to 55.9% wet weight. As a consequence the protein content was reduced from 32.6% to 22.1% wet weight. The lipid content revealed an identical decrease from 20.2% to 10.9% wet weight. It becomes apparent from the high variation of the results, especially for samples with water contents between 47 and 56% wet weight, that other factors besides the salt content are responsible for a significant variability in gross composition. The stage of gonadal development as well as the number of previous spawnings have considerable influences upon the egg-size and its composition.

Restoration of the European sturgeon, Acipenser sturio L., 1758 in Germany: Effect of Different Feed Items on Specific Growth Rates of Large Juvenile Fish

23 Acipenser sturio juveniles (51 months old, median total length 80 cm, median weight 1950g) of a 1995 artificially reproduced Fl population form the Cemagref, Bordeaux were used for feeding experiments for 28 weeks. Fish were kept in two experimental tanks (5.5 m 3 ) at 20.5 °C and natural photoperiod in freshwater. During the first 16 weeks fish were fed three frozen components (3C), small marine fish (FSMF), small chironomids (FSC) and large chironomids (FLC) except for 3 two weeks periods when the 3 single components were tested: the highest median specific growth rates (MSGR) were obtained with the FLC feed (0.43% bw/d). From weeks 17 to 28 FLC feed was offered except for 2 two weeks periods: a feeding experiment was performed with FLC plus frozen pieces of smelt (MSGR -0.18% bw/d) and a starvation experiment (MSGR -0.32% bw/d). A. sturio showed a high variation in both the MSGR and the individual SGR values thus limiting the interpretation of these quantitative results.

Larval Electroreceptors in the Epidermis of Mormyrid Fish: II. The Promormyromast

Promormyromasts were found in the epidermis of the head of the larvae of five species of mormyrids bred in captivity. The promormyromast is a larval electroreceptor belonging to the specific lateral line system. In 12‐day‐old larvae this electroreceptor is characterized by a single sensory cell and two types of accessory cells. One type of accessory cell has dark cytoplasm, few microtubules, and contacts the sensory cell directly, whereas a second type has pale cytoplasm, many microtubules, and forms an outer layer not directly in contact with the sensory cell. This second type is referred to as a long pyriform accessory cell. This assembly of cells is situated below an intraepidermal cavity filled with acid polysaccharides. The bordering epidermal cells extend microvilli into the intraepidermal cavity. The apexes of the sensory cell, and of the two types of accessory cells, also open into the intraepidermal cavity but bear no microvilli. The promormyromast is innervated by an unmyelinated sensory nerve fiber passing through the basal membrane, which then splits into several branches between the accessory cells. These branches contact the periphery of the sensory cell with terminal boutons. At the site of each contact a ribbon‐like structure surrounded by vesicles is present in the cytoplasm of the sensory cell. In older larvae of Campylomormyrus cassaicus, membrane foldings develop at the periphery of the pyriform accessory cells and accessory cell staining properties change just before transformation to become a mormyromast. The functional role of the promormyromast of the larval mormyrids is discussed. J. Comp. Neurol. 501:810–823, 2007.