Variation In Year-class Strength Research Articles

Causes of Variations in Year-Class Strength of Amur Pink Salmon, Oncorhynchus gorbuscha (Walbaum)

Causes of Variations in Year-Class Strength of Amur Pink Salmon, Oncorhynchus gorbuscha (Walbaum)

Early life growth is affecting timing of spawning in the semelparous Barents Sea capelin (Mallotus villosus)

Capelin (Mallotus villosus) is a forage fish and a key species in the Barents Sea (BS). The BS capelin are semelparous and hence only spawn once along the north coasts of Norway and Russia before they die. The age at spawning ranges from 2 to 5 years and the spawning season peaks in March/April but starts in February and lasts until June, and the causes of the variability in timing of spawning are not well understood. Here, we aimed to find out whether early growth is associated with the timing of spawning in BS capelin, both on the individual and population level, and if there is an association between early life growth and the spatial distribution at the nursery areas and feeding grounds. For the analysis, we used an extensive dataset comprising >150000 otolith growth zone measurements carried out during surveys from 1976 to 2019 both from the spawning and feeding areas. The data from the feeding area showed that capelin with good first-year growth were found in the productive north-west part of the Barents Sea at both age 1 and 2, while capelin with relatively poor first-year growth were typically found in the south-east Barents Sea. The data from the spawning area showed on the individual level that capelin with good first-year growth tend to spawn both at a younger age and earlier in the season. The capelin spawning late in the season were also generally smaller than early spawners. On the population level, a contradictory pattern was observed where the proportion of maturing capelin at age 2 and 3 was negatively correlated with first-year growth indicating that the great variability in year-class strength masks the general effect found at the level of individual fish. Furthermore, first-year growth was positively associated with the abundance of 1-year-old capelin indicating that rapid growth early in life enhances recruitment. On the other hand, first-year growth was strongly negatively correlated with third-year growth suggesting an increased effect of density-dependent growth with age and/or compensatory growth, or reduced growth linked to earlier maturation. In sum, our results show that the first-year growth affects growth, maturation processes, and timing of spawning later in life, thus potentially strongly influencing capelin population dynamics.

Drivers of walleye recruitment in Minnesota’s large lakes

Fish recruitment is complex and difficult to predict. Data-driven approaches show promise for predicting recruitment and understanding its drivers. We used a random forest model to infer relationships between year-class strength and 17 variables describing potential recruitment drivers across 30+ years of walleye (Sander vitreus) data from Minnesota’s nine largest inland lakes. Our model explained 20% of the variation in year-class strength overall, with predictive performance varying among lakes (–8% to 37% explained variance). Of the variables analyzed, degree-days during the first year of life and first winter severity were the most important for predicting recruitment, with relatively weak year classes predicted to occur with cold first growing seasons and severe first winters. Other thermal variables were secondarily important predictors of year-class strength. Predicted year-class strength was positively related to stock size and stocking and negatively related to the presence of invasive species; however, these variables were less important than thermal variables. Our results indicate that thermal conditions in early life can have a substantial impact on walleye recruitment and highlight the potential for differing recruitment drivers and dynamics among lakes.

Recruitment variability in Baltic flounder (Platichthys solemdali) – effects of salinity with implications for stock development facing climate change

The brackish water Baltic Sea is strongly influenced by irregular saline water inflows. Fewer inflow events since the 1980s have resulted in a decrease in salinity, and will according to predicted climate change continue to decrease. The newly described Baltic flounder Platichthys solemdali has adapted to reproduction at low salinity conditions since it colonized the Baltic Sea 7000 years BP; in the area studied (ICES SD 3d 28.2) spawning occurs at 3–20 m depth at ca 7 psu. Here we monitored variability in year-class strength as newly settled 0-gr fish in three coastal nursery areas, and compared obtained recruitment indices with prevailing temperature and salinity conditions. 0-gr abundance indices varied considerably between years, from 1 to 90, 10–296 and 17–86 at the respective sampling site, and showed strong accordance with the age structure of the adult stock. Variability in temperature showed no effect, but stronger and weaker year-classes respectively were related to variability in salinity in the range 6.6–7.1 psu with stronger year-classes at >6.8 psu. This coincides with variability in spermatozoa motility, fertilization rates and early egg development at different salinities and suggests that the year-class strength may be set already at the egg stage. Thus, only small changes in salinity at spawning may affect reproductive success and ultimately stock development. The potential effects on stock abundance and distribution facing climate change call for changes in future fishing patterns as well as adaptations in management to prevent stock collapse.

Effects of an Extreme Flood Event on Federally Endangered Diamond Darter Abundances

Extreme flood events can substantially affect riverine systems, modifying in-stream habitat and influencing fish assemblages and densities. Rare species are especially vulnerable to these disturbance events because of their small population size and often reduced phenotypic heterogeneity. In June 2016 the lower Elk River in West Virginia experienced severe flooding, resulting in a peak discharge that exceeded the 0.005 annual exceedance probability (>200 y flood) in the main stem. We obtained pre-flood and post-flood population count data and estimated abundances for one cohort of the federally endangered Diamond Darter (Crystallaria cincotta) at 15 sites. While both the total count data and total estimated abundance decreased following the flood, our analyses did not indicate the extreme flood event strongly impacted Diamond Darter abundance. This indicates individuals are able to withstand high velocities and resist displacement or mortality. In addition site-level abundances were estimated at three sentinel sites during 2015 and 2016 using a multinomial N-mixture model that accounted for variation in detectability resulting from water temperature. Mean estimated abundance varied among the three sites and between the 2 y. Our results suggest there is substantial variation in year-class strength between the two cohorts we sampled. It is suggested that survey efforts at established sentinel sites be continued on an annual basis in order to help determine factors influencing year-class strength.

Mechanisms driving recruitment variability in fish: comparisons between the Laurentian Great Lakes and marine systems

AbstractIn his seminal work, Hjort (in Fluctuations in the great fisheries of Northern Europe. Conseil Parmanent International Pour L'Exploration De La Mar. Rapports et Proces-Verbaux, 20: 1–228, 1914) observed that fish population levels fluctuated widely, year-class strength was set early in life, and egg production by adults could not alone explain variability in year-class strength. These observations laid the foundation for hypotheses on mechanisms driving recruitment variability in marine systems. More recently, researchers have sought to explain year-class strength of important fish in the Laurentian Great Lakes and some of the hypotheses developed for marine fisheries have been transferred to Great Lakes fish. We conducted a literature review to determine the applicability of marine recruitment hypotheses to Great Lakes fish. We found that temperature, interspecific interactions, and spawner effects (abundance, age, and condition of adults) were the most important factors in explaining recruitment variability in Great Lakes fish, whereas relatively fewer studies identified bottom-up trophodynamic factors or hydrodynamic factors as important. Next, we compared recruitment between Great Lakes and Baltic Sea fish populations and found no statistical difference in factors driving recruitment between the two systems, indicating that recruitment hypotheses may often be transferable between Great Lakes and marine systems. Many recruitment hypotheses developed for marine fish have yet to be applied to Great Lakes fish. We suggest that future research on recruitment in the Great Lakes should focus on forecasting the effects of climate change and invasive species. Further, because the Great Lakes are smaller and more enclosed than marine systems, and have abundant fishery-independent data, they are excellent candidates for future hypothesis testing on recruitment in fish.

Recruitment in the Barents Sea, Icelandic, and eastern Newfoundland/Labrador capelin (Mallotus villosus) stocks

Capelin (Mallotus villosus) is a short-lived, coldwater, pelagic species that occurs in large populations in the Barents Sea, around Iceland and in the Newfoundland–Labrador area in the North Atlantic. Most individuals spawn only once at age three or four and die shortly after spawning. The commercial fisheries for capelin in the three areas are prosecuted on the pre-spawning and spawning age groups and knowledge of recruitment to the spawning stock and factors affecting recruitment are important in the management of the fisheries. Herein, we review the state of knowledge regarding recruitment in the three areas and factors that affect recruitment.Year class strength of capelin is fixed fairly early in life; at 0-group stage in the Barents Sea, by age 1 or earlier for Icelandic capelin, and within 2weeks of hatching for the beach spawners in the Newfoundland area. No obvious single factor, common to the three stocks, has been proven to exert a significant influence on recruitment. In the Barents Sea, juvenile herring prey heavily on capelin larvae and when the estimates of the abundance of these juvenile herring were accounted for in the capelin stock–recruitment analysis, the fit in the stock–recruitment relationship improved. In Iceland, there is no reason to implicate predation as a dominant factor, although detailed studies have not been carried out. In the Newfoundland and Labrador area earlier studies suggested two factors were important for recruitment success: the frequency of onshore winds at a critical period during the residence of newly hatched larvae in the beach gravel and the presence of warm waters during the early larval phase in the pelagic environment. With further testing using more data, the temperature factor was found not to be significant but the frequency of onshore winds still explained a significant portion of the variation in year class strength. There appears to be no significant biological influence, such as predation, that has influenced recruitment in this area.

The effects of temperature change on the hatching success and larval survival of largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieu

In this study, the effects of abrupt temperature change on the hatching success and larval survival of eggs, yolk-sac larvae (YSL) and larvae above nest (LAN), for both largemouth bass Micropterus salmoides and smallmouth bass Micropterus dolomieu were quantified. Temperature had a significant effect on hatching success and time to 50% mortality, with large heat shocks causing accelerated mortality. The temperature changes shown to influence survival of all life stages, however, were beyond what is typically experienced in the wild. Micropterus salmoides had greater egg hatching success rates and increased survival rates at YSL and LAN stages, relative to M. dolomieu. Additionally, egg hatching success and survival of LAN varied across nests within the study. These findings suggest that temperature alone may not account for variations in year-class strength and may emphasize the need for protection of the nest-guarding male Micropterus spp. to ensure recruitment.

Do recent age structures and historical catches of mulloway,Argyrosomus japonicus(Sciaenidae), reflect freshwater inflows in the remnant estuary of the Murray River, South Australia?

Patterns of annual freshwater flows in the Murray River and recruitment of mulloway, Argyrosomus japon- icus (Scianidae) were reviewed in terms of recent age structures and historical catches and CPUE. Age distributions from the nearshore marine fishery were dominated by the 1993 age class which comprised 35% and 41% of 2001 and 2002 catches, respectively. In 1993 annual freshwater inflow was 2.4 times the 25 year average. Freshwater inflow explained 28% and 35% of the variability in year class strength in the nearshore marine fishery in 2001 and 2002, re- spectively. Over 80% of the current South Australian commercial catch of mulloway comprises juveniles taken from the remnant estuary of the Murray River. Our results suggest that recent low levels of recruitment in South Australia's fish- eries for A. japonicus may reflect low fresh water inflows since 1993. Since 2000, southern Australia has experienced the worst drought in recorded history and management strategies for this fishery must take into account effects of both environmental factors and fishing mortality on this vulnerable sciaenid population. We suggest that the population of A. japonicus located about the Murray River system is estuarine dependent, that the estuary provides important refuge for juveniles, and that strong year classes, or their absence, may be related to freshwater inflow to this environment. We also suggest that age distributions of this apex predator may provide an indicator of environmental health for the Murray River estuary.

Temporal changes in lipid condition and parasitic infection by digenean metacercariae of young-of-year common sole Solea solea (L.) in an Atlantic nursery ground (Bay of Biscay, France)

Little attention has been paid to flatfish parasitism as a potential factor of variation in year-class strength. The aim of this study was to evaluate the relationship between parasitic infection and nutritional condition of young-of-year (YOY) common sole. Sole were collected monthly from May (early settlement) to November 2004 in a mussel pole culture area, using a push-net or a trawl, and dissected for parasite identification and counts. Total lipids were extracted from the whole body and lipid classes quantified. Triacylglycerols to sterols (free form) ratio (TAG/ST) was used as an index of nutritional status. Digenean metacercariae infection, characterized by two dominant genera, displayed clear seasonal dynamics and the highest records yet reported for sole. Prevalence (P%) and mean abundance (Ab ± SE) increased for Timoniella spp. (Acanthostomidae) from May (P% = 22%; Ab = 1.9 ± 1.1) to August (P% = 100%; Ab = 51.2 ± 10.4), and for Prosorhynchus spp. (Bucephalidae) from June (P% = 8%; Ab = 0.1 ± 0.0) to August (P% = 100%; Ab = 19.5 ± 1.7). Epidemiological values reached a plateau after August. In this shellfish-dominated nursery area, YOY sole accumulated metacercariae in the presence of the first intermediate hosts which live in the same habitat. Total lipids decreased significantly from May to June and then remained constant until autumn, indicating a change in the proximate composition of YOY during this high growth period. Mean (± SD) TAG/ST ratio increased from very low values in May–July (0.10 on average) to a peak in September (0.57 ± 0.31) and then fell back to values as low as before. YOY sole only stored energy as TAG prior to autumn. No relationship was found between YOY parasitic infection levels and lipid condition indices. However, the poor nutritional status and heavy infection level observed in November could dramatically lower over-winter survival of the YOY remaining in this shallow part of the nursery.

The influence of life history dynamics and environment on the determination of year class strength in North Sea herring (Clupea harengus L.)

AbstractThe inter‐annual variability in year class strength (1976–2000) of North Sea herring (Clupea harengus) was investigated using Paulik diagrams based on survey data and Virtual Population Analysis. The herring life cycle was split into five stages: spawning stock biomass (SSB), egg production, larvae, fish with 0 winter rings on the otolith (0‐wr), 1‐wr and 2‐wr. Surveys were used as indices and Paulik analysis revealed relationships between stages. In 80% of the years, year class strength reflected SSB. Poorer than expected year classes were determined during the larva to 0‐wr phase, whilst stronger than expected year classes were apparently determined during the 0‐wr to 1‐wr stage. There was no clear relationship between survival of young stages of herring and the abundance of Calanus finmarchicus but the year class strength of 0‐wr and 1‐wr had a negative relationship to bottom water temperature. Lower sea water temperatures in the North Sea are associated with higher Calanus abundance. The analysis shows that the strength of aberrant year classes of North Sea herring is determined between the pelagic larval and the juvenile stages.

Spatial Patterns Emphasize the Importance of Coastal Zones as Nursery Areas for Larval Walleye in Western Lake Erie

Lake Erie walleye Sander vitreus exhibits significant interannual variability in year-class strength. Recent research revealed the importance of larval growth and survival rates in determining walleye year-class strength in western Lake Erie, indicating that spatial and temporal overlap of larvae with good habitat conditions (e.g., abundant prey, warm waters) promoted walleye growth and survival. To assess the spatial overlap between walleye larvae and habitat parameters (water depth, temperature, water clarity, prey density) in western Lake Erie, we evaluated the spatial distribution of walleye larvae and these habitat parameters with intensive sampling at 30 to 36 sites during spring 1994–1999. We analyzed spatial relationships among pelagic walleye larvae and various habitat attributes using a geographic information system and principal components analysis. Larval walleye density was consistently highest at nearshore sites during all years and showed a high degree of spatial overlap with high ichthyoplankton density, and warm water temperatures. Larval walleye density was negatively associated with water depth and water clarity. Two principal components represented 79.6% of the total variability in site attributes. Principle components analysis supported our spatial analysis by graphically separating sites into distinct groups based on larval walleye density and habitat attributes. These analyses indicated that similar relationships between larval distribution and habitat attributes occur each year, emphasizing the importance of nearshore coastal zones as nursery areas for walleye.

High levels of spatial and temporal recruitment variability in the temperate sparid Pagrus auratus

Knowledge of spatial and temporal variation in the abundance of early life stages is important to developing an understanding of juvenile recruitment processes and, ultimately, the dynamics and demographics of fish populations. In Victoria, south-eastern Australia, snapper, Pagrus auratus, supports an important fishery characterised by high variability in year-class strength. We investigated spatial and temporal variation in the recruitment of small juvenile (0+) snapper by monitoring their abundance in four bay and inlet nursery areas during four consecutive summer/autumn recruitment periods (2000–2003). We found considerable spatial variability in the abundance of recruits, both within and among inlets. Interannual recruitment variation, however, differed among inlets. Recruitment into the largest nursery area and most important fishery, Port Phillip Bay, varied by ~10-fold across years, whereas variation for the other inlets was low or not significant. There were also clear differences in size distributions of recruits among both inlets and years. Strong recruitment in Port Phillip Bay during 2001 was related to a longer period of spawning and successful larval settlement and was associated with above average water temperatures. Future monitoring of 0+ recruitment in Victoria and studies of the recruitment processes should be specific to individual nursery areas.

Egg production of Baltic cod (Gadus morhua) in relation to variable sex ratio, maturity, and fecundity

Observed fluctuations in relative fecundity of Eastern Baltic cod (Gadus morhua L.) were related to food availability during the main feeding period and were used to develop a predictive model that explained 72% of the interannual variations in fecundity. Time series of sex ratios, maturity ogives, and relative fecundity were combined with mean weights-at-age and stock sizes from an analytical multispecies model to estimate the potential egg production (PEP). Relationships between PEP and independent estimates of realized daily and seasonal egg production from egg surveys were highly significant. The difference between estimates of potential and realized seasonal egg production was of a magnitude corresponding to the expected loss of eggs as a result of atresia, fertilization failure, and early egg mortality. The removal of interannual variability in sex ratio, maturity, and fecundity on estimates of PEP deteriorated the relationships in all three cases. PEP proved to be superior to spawning stock biomass as measure of the reproductive potential in a stock-recruitment relationship of Eastern Baltic cod. PEP in combination with the reproductive volume explained 61% of the variation in year-class strength at age 2.

What Does Mt. Pinatubo Have to Do with Walleyes?

The eruption of Mt. Pinatubo in the Philippine Islands during June 1991 was followed by a global temperature decline of 1°F through 1993. May to July temperatures in Minnesota during 1992 and 1993 were 3°F below the 1979–1998 average. The 1992 and 1993 year-classes of walleyes Stizostedion vitreum from Minnesota's nine largest walleye lakes, which are gillnetted annually, were the two weakest since 1979. Mean June temperature explained more than 40% of the variation in year-class strength. Mean statewide walleye gill-net catch per unit effort (CPUE) from lake surveys increased at a rate of 1.3% annually after 1979 and was positively related to rising June temperatures, which explains 57% of the variation from lakes in glacial drift and 28% of the variation from Precambrian shield lakes. Walleye fry stocking was also positively related to increased walleye gill-net CPUE, but fingerling stocking was not. Chaotic events, such as large volcanic eruptions, may negatively affect walleye recruitment, and these effects may be evident to anglers several years later.

Trends in interannual yield variation of reservoir fisheries in Bangladesh, with special reference to Indian major carps

AbstractInterannual variation in yield per species from a freshwater reservoir in Bangladesh was analysed by using long‐term landing data. Each species was categorized according to characteristics in interannual yield variation. The characteristics were defined with respect to size of the interannual variation in terms of absolute and relative variation around the mean yield and long‐term trends in catch levels. Annual variation was directly (t18 = 7.84; P < 0.01) related to the yield and can serve as a good indication of the status of the fishery when catches are low. The target fish, Indian major carps (IMC) Labeo rohita, Labeo calbasu, Catla catla and Cirrhinus mrigala, showed large variation in annual yield. This might have occurred because of variation in annual recruitment. Illegal use of gears is mostly related to variation in year class strength, and is assumed to be the most important source of change of stock variation. An increase in catch rate was recorded for Corica soborna. Such a stable combination may only be possible for a certain period without a long‐term trend. Trends in the yield rate of the major fish category for three decades reveal a sharp decline in the catch of IMC. In contrast, small clupeid‐like C. soborna have the highest yield rate. Fishing gears can play a crucial role in this fishery. Fisheries managers must always be alert when issuing fishing licenses and on the control on gears. The conclusion is that the IMC fishery of Kaptai Reservoir is an overexploited fish stock. In this fishery, the same gears are usually employed indiscriminately for different species and size classes. To stabilize the fishery, the existing policy needs to be strengthened and more effectively implemented. Also, some fishing gears and traps need to be restricted. An effective stock protection policy is suggested. Further study on catch size of individual species is essential prior to the development and implementation of any management scheme.

Low-Temperature Tolerance of Age-0 Atlantic Croakers: Recruitment Implications for U.S. Mid-Atlantic Estuaries

Age-0 Atlantic croakers Micropogonias undulatus overwinter in Mid-Atlantic Bight (MAB) estuaries, where interannual variability in year-class strength is pronounced. Previous researchers have hypothesized that this variability is the result of cold-induced winter mortality of age-0 Atlantic croakers within estuarine nurseries. We tested this hypothesis by means of laboratory experiments designed to (1) estimate the effects of winter severity and duration on the probability of survival of age-0 fish, (2) determine the relationship between body size and cold tolerance, and (3) examine the effects of temperature decline rate and salinity on susceptibility to low-temperature mortality. Age-0 Atlantic croakers (15–65 mm standard length, SL) were collected from Delaware Bay during October, acclimated to 8°C in the laboratory, and then cooled (1.0°C/d) to treatment levels of 1, 3, 5, or 7°C that were designed to simulate varying levels of winter severity in MAB estuaries. Survival ranged from 0% at 1°C to 99.3% at 7°C, increasing dramatically between 3°C (1.3%) and 5°C (86.8%). Cold tolerance was size dependent, with smaller individuals surviving significantly longer than larger ones. The rate of temperature decline (1.0°C/d versus 0.2°C/d) did not affect survival at lower winter temperatures. Atlantic croakers remained intolerant of temperatures of 3°C or less even when these temperatures were approached gradually. Cold tolerance increased with increasing salinity, implying that the risk of overwinter mortality varies spatially along an estuarine salinity gradient. The mean survival duration at 3°C increased from 5.9 d at 2‰ to 14.7 d at 10‰ and to 17.1 d at 26‰. Our findings provide empirical support for the hypothesis that cold-induced overwinter mortality regulates the year-class strength and recruitment success of Atlantic croakers in the Mid-Atlantic Bight.

THE INFLUENCE OF WATER TEMPERATURE AND FLOW ON YEAR CLASS STRENGTH OF TWAITE SHAD (ALOSA FALLAX FALLAX) FROM THE RIVER SEVERN, ENGLAND.

Year class strength (YCS) was estimated for the 25 year period between 1972 and 1996. The index (YCS) was based on the mean number of six year old female fish caught per tide (1972-1991) in a net fishery at the start of the freshwater phase of their spawning migration, or predicted from juvenile data (1992-1996). The variation in recruitment as measured by the coefficient of variation for the 20 year period (1972 to 1991) was 124.5 %. There was good spawning success in 1976 and 1989 and particularly poor recruitment in the periods 1977 to 1981 and 1985 to 1988. Since 1990 recruitment has remained relatively stable. Water temperature was positively correlated to YCS with mean July temperature explaining the greatest proportion of the variance in YCS (67 %), followed by August (48 %), June (31 %) and October (20 %) ; May and September temperatures did not significantly explain the variation in YCS. Flow in the months June to August were significantly inversely correlated with YCS, with the greatest proportion of the variability explained by August flows (41 %), followed by flows in July (36 %) and June (25 %). Flows in May, September and October were not significantly correlated with YCS. Combining environmental variables in a multiple regression indicated that mean daily temperature between June and August accounted for 77.1 % of the variability in year class strength. The inclusion of flow did not increase significantly the amount of variation explained.

Relationships between Annual Variations in Reservoir Conditions and Age-0 Largemouth Bass Year-Class Strength

Variable recruitment has been documented in populations of largemouth bass Micropterus salmoides throughout the species' range. Shoreline electrofishing was used from 1988 to 1998 to assess the magnitude of variations in year-class strength of largemouth bass at the time cohort mean lengths reached 50 mm in Jordan Lake, North Carolina. Concurrent measures of adult stock characteristics, dynamics of larval and juvenile shad Dorosoma spp., lake elevation dynamics, and seasonal air temperatures were used to test published findings concerning factors that influence largemouth bass year-class strength. Catch rates of juvenile largemouth bass varied fourfold from 1988 to 1995. No significant relationships were observed between largemouth bass year-class size and spawning stock size, shad dynamics, or variations in lake elevations. However, late-winter and early spring air temperatures explained a significant amount of the observed variation in catch rates of age-0 largemouth bass. Abundance of yearling largemouth bass in early spring also varied significantly among years and was significantly correlated with differences in age-0 abundance at 50 mm the previous year, suggesting that differences in abundance established early in the life history of largemouth bass play a significant role in recruitment variation. Although relative year-class strength varied predictably with prespawn air temperatures, long-term trends indicated a decline in relative year-class strength over the course of 11 years. Broadscale factors, such as temperature, may introduce variation about the expected mean year-class size of largemouth bass in Jordan Lake, which can be further modified by variations in local conditions.

Recruitment in flatfish, with special emphasis on North Atlantic species: Progress made by the Flatfish Symposia

Van der Veer, H. W., Berghahn, R., Miller, J. M., and Rijnsdorp, A. D. 2000. Recruitment in flatfish, with special emphasis on North Atlantic species: Progress made by the Flatfish Symposia. – ICES Journal of Marine Science, 57: 202–215. In summarizing the main results on recruitment that emerged from the series of Flatfish Symposia, two aspects were distinguished: mean level and interannual variability. Recruitment to a stock appears to be related to the quantity of juvenile nursery habitats, suggesting that either larval supply or the carrying capacity of the nurseries is the limiting factor. However, available information on growth of 0-group flatfish suggests that the carrying capacity of nursery areas is never reached. Variability in year-class strength is generated during the pelagic egg and larval stage, probably by variations in the hydrodynamic circulation and in the mortality rates of eggs and larvae. Density-dependent processes seem to occur only during the juvenile stages, particularly in respect of growth. However, no impact on recruitment variability has been found. Density-dependent mortality during the phase shortly after settlement dampens the interannual recruitment variability. There is no evidence of densitydependent effects in the adult phase at present, but these may have been important at lower levels of exploitation. The importance of the factors determining recruitment vary not only among species, but also over the species’ range. It is suggested that damping processes can only occur in the demersal stage, implying that variability in year-class strength can only decrease in fish species with a demersal stage. If true, ultimate variability in recruitment in fish species will be related to the relative duration of the pelagic and demersal stages. 2000 International Council for the Exploration of the Sea