Rearing Conditions Affect Chemosensory Preferences in Young Cichlid Fish

Offspring of the Midas cichlid, Cichlasoma citrinellum, reared with both parents showed a chemosensory preference for their mother over their father on the first day of free-swimming and no clear preference for either parent thereafter. Fry reared in isolation preferred their mother; this preference appeared slightly stronger and longer-lasting in fry reared with only their mother. The preference was reversed in fry reared with only their father.     

The chemosensory responses of young cichlid fish to parents and predators

The chemosensory responses of the fry of the Midas cichlid, Cichlasoma citrinellum (Pisces: Cichlidae), were tested in a Y-maze during their first 11 days of free-swimming. Fry preferred all conspecific females to blank water but did not distinguish between their own mother, another mother or a non-parental, adult female. Fry preferred females to males. Similar results were obtained for males. Fry preferred their siblings and (unrelated) juveniles to blank water. However, while they failed to show a preference between siblings and their mother, they preferred their mother to juveniles. Fry were able to any of three heterospecifics (C. managuense, C. nigrofasciatum and the allopatric Sarotherodon mossambicus). The chemosensory responses demonstrated in the Y-maze are hypothesized to aid the fry in staying near the centre of the guarded territory and away from predators, particularly under murky or nocturnal conditions.     

To call or not to call: parents assess the vulnerability of their young before warning them about predators

Communication about predators can reveal the effects of both conspecific and heterospecific audiences on signalling strategy, providing insight into signal function and animal cognition. In species that alarm call to their young, parents face a fundamental dilemma: calling can silence noisy offspring and so make them less likely to be overheard, but can also alert predators that young are nearby. Parents could resolve this dilemma by being sensitive to the current vulnerability of offspring, and calling only when young are most at risk. Testing whether offspring vulnerability affects parental strategy has proved difficult, however, because more vulnerable broods are often also more valuable. We tested experimentally whether parent white-browed scrubwren, Sericornis frontalis, assessed brood noisiness when alarm calling near nests. When a model predator was nearby, parents gave more alarm calls when playbacks simulated noisy broods, yet brood noisiness did not affect adult calling when only a control model was present. Parents were therefore sensitive to the tradeoff between silencing young and alerting predators to the presence of nests. Our study demonstrates that receiver vulnerability can affect signalling decisions in species other than primates.     

Effect of environment on the adaptive abilities of fry roach Rutilus rutilus (L.) (Cyprinidae) during early ontogenesis

In recent years a considerable decrease in the abundance of predatory fishes has been observed in spawning tributaries of the Rybinsk Reservoir caused by their intensive catching. The lack of encounters with predators before the downstream migration of young fish hampers the development of necessary skills of defensive behavior in the absence of predation experience. As a result, after downstream migration, the juveniles are incapable of adapting to the predation pressure in the reservoir and are subjected to intensive elimination. The adaptive potential of the roach Rutilus rutilus L. was experimentally studied in siblings raised from the larvae to the late fry stage both in the presence and absence of a predator. It has been found that the fry that was raised under different conditions differed in their adaptive potential in new environment conditions.     

First access to territorial space and exposure to strong predation pressure: A conflict in early emerging Atlantic salmon (Salmo salar L.) fry

Summary This work aimed to elucidate conflicting factors that may explain the narrow and synchronous emergence in salmonids. Fry are highly vulnerable to predation and stand a better chance of surviving if they emerge synchronously. On the other hand, fry that leave the gravel first should increase their chance of obtaining one of the limited number of feeding territories. The risk involved in early emergence for Atlantic salmon fry was evaluated by exposing them to predatorySalmo trutta. Yolk sac alevins were incubated in an artificial redd in order to catch them by their time of emergence. Early-, peak- and late-emerging fry were then successively marked and transferred to flume tanks in which the predators were either present from the start or not introduced until all fry had been added. When the predators were initially present, the predation pressure differed depending on the time of fry emergence, resulting in survival rates of 11.6, 44.9 and 51% in early-, peak- and late-emerging fry, respectively. By assuming that the predation rate of the three emergence groups was dependent both on time of emergence and fry density the survival rates were calculated to be 7.5, 41.1 and 53.5% in groups I, II and III, respectively. These figures corresponded well to the observed rates. When the predators were added after completed emergence the resulting survival rates were 56.9, 39.7 and 25.2% in early-, peak- and late-emerging fry, respectively. Thus, predation after complete emergence gave a survival probability that varied across the three emergence-date groups, despite being exposed to a predator during the same number of days. The presence of fish predators in combination with a limited territorial space seems to make both early and late emergence hazardous and favours synchronous swimming movements.     

Parental care and the ontogeny of predator-avoidance in two species of centrarchid fish

Stamps (1978) suggested that the type and extent of parental care can influence the behavioural ontogeny of the offspring. Largemouth bass (Micropterus salmoides) males guard the fry for up to a month after they leave the nest while rock bass (Ambloplites rupestris) males guard the fry only until they leave the nest. In laboratory experiments, naive largemouth fry displayed reduced predator-avoidance responses to large and small predators during their first 3 weeks of free-swimming. Rock bass fry, on the other hand, avoided predators throughout the study period. From 6 weeks (largemouth) and 5 weeks (rock bass) of age the fry showed a significantly greater response to the large predator than the small predator. The responses of laboratory-reared largemouth bass fry to the small predator changed from avoidance to association as the fry grew. The size of the fry relative to predator size was found to be important.     

Biology of young grey mullet, Mugil cephalus L., populatioins in a coastal lagoon in Sri Lanka

The biology of young grey mullet, Mugil cephalus L. populations of a coastal lagoon—the Negombo Lagoon on the West Coast of Sri Lanka was studied from October 1976 to March 1978. The relative abundance of fry were found to vary from month to month, with peaks occurring in December-January, May and September-October, coinciding with the end of rainy seasons. It was found that the young fry tended to migrate into the lagoon at a size of about 10–20 mm, most frequently between 15–20 mm in length, by which time they were fully scaled. The young fry after a period of growth in the shallow areas of the lagoon, when they reached a size of 30–35 mm, emigrated into the deeper waters of the lagoon. The rate of growth, as computed from length frequency distribution studies, was found to be around 0.24 mm day⁻¹. Results of diurnal surveys showed a movement of the larger size fry into shallower waters during the hours of darkness and this movement appears to be independent of the tidal cycle. It is postulated that the cause of this migration is to escape from predators, chiefly piscine. Investigations of the meristic characters indicated that the young fry which migrated into the lagoon at different times of the year probably originated from a single spawning stock.     

Survival of broods under parental care and parental roles of the cichlid fish,Lamprologus toae,in Lake Tanganyika

The survival of offspring guarded by both parents and by only the female parent who remained after experimental removal of the male parent was observed in the cichlid fishLamprologus toae in its natural habitat. The number of offspring guarded by both parents abruptly decreased during the period from eggs through postlarvae. It became stable for low rate of mortality when the offspring were between 0.5 and 1.8 cm in body length. The number again decreased this time gradually after attaining 1.8 cm in body length. Mortality seemed to be caused by predation. Experimental removal of the male parent at any larval stage caused the decrease of offspring by predatory fishes. It seems that both parents are indispensable to protect the offspring against predators under natural conditions.     

Cannibalism as the cause of an ontogenetic shift in habitat use by fry of the threespine stickleback

Summary In Crystal Lake, British Columbia, small fry (15 mm SL) of the threespine stickleback (Gasterosteus aculeatus) are concentrated in vegetation while larger fry are not. Because fry in all size classes feed primarily on zooplankton, even when in vegetation, we hypothesized that size-limited predation was responsible for the observed shift in habitat use with size. The major predators on fry in Crystal Lake are adult threespine stickleback, the water scorpion, Ranatra sp. (Hemiptera: Nepidae), backswimmers, Notonecta spp. (Hemiptera: Notonectidae), and dragonfly naiads of the genus Aeshna (Odonata: Aeschnidae). On the basis of distribution and hunting behavior we excluded the insects Ranatra sp., and Notonecta sp. as causal agents for this shift in resource by fry in water >0.25 m deep. Ranatra was found almost exclusively near the shoreline in water <0.25 m deep, and both insects hunted primarily as ambush predators within vegetation. Such predators seemed more likely to drive vulnerable fry from vegetation than to restrict them to it. In contrast, Aeshna naiads and adult stickleback frequently hunted outside of vegetation. In prey preference experiments the naiads did not show the decline in predation efficiency on fry >15 mm SL that would be expected if size-limited predation by this insect was responsible for the observed shift in resource use by fry. Adult stickleback only fed on fry <15 mm SL, and in an experimental situation, consumed fry at a rate 10 times greater than that exhibited by any of the insects. Predation experiments demonstrated that small fry (11–15 mm) spent more time in vegetation in the presence of adult conspecifics than they did in control pools, as would be expected if size-limited cannibalism caused small, vulnerable fry to be restricted to vegetation. Fry >15 mm SL were found outside of vegetation more often than in control treatments. The probable cause of this result is that adults become aggressive toward fry at this size, and often could be seen chasing large fry from vegetation during the experiments. Dragonfly naiads (Aeshna spp.) spent most of their time in vegetation in the experimental pools. Both size classes of fry spent less time in vegetation in the presence of dragonfly naiads than they did in control treatments, an apparent reflection of their similar vulnerabilities to these naiads. The presence of vegetation in pools reduced predation rates by adult stickleback on small fry. Because the experiments presented here indicate that fry are capable of rapidly assessing predation risk and of altering their behavior adaptively, we conclude that small fry occupy vegetation as a refuge from cannibalism. Once fry have reached the size-threshold at which they are no longer vulnerable to adult conspecifics they are able to forage farther from vegetation thereby reducing risk of predation by insects in vegetation and possibly acquiring more abundant food resources.     

Mate choice in the monogamous and polychromatic Midas cichlid, Cichlasoma citrinellum

Choice of mate in the Midas cichlid, Cichlasoma citrinellum (Günther, 1864), has been examined from a number of perspectives, proceeding from general species attributes to individual differences.
The Midas cichlid discriminates against closely-related, highly-similar Cichlasoma , and recognizes sex, probably chemically in both cases. It is polychromatic and changes from the normal (N) pattern to gold (G) or even white at almost any age. Gold colouration inhibited attack and made it difficult to mate an N female with a G male; the inhibiting effect was cancelled by using a larger N female.
Because the Midas cichlid is also biparentally monogamous, early experience could easily influence later choice of mate. G females of G × G parents preferentially spawned adjacent to G males; N females of N × N spawned mainly before N males. N females of G × G pairs, however, showed no preference.
Groups of females were reared by all four combinations of two colours and two sexes, or by no parents. Females reared by identically pure-coloured parents responded most to males of those colours, whereas females raised by mixed-colour parents did not discriminate. Females reared without parents preferred N males. Regardless of their own or their parents' colour, juveniles spent more time with N juveniles and behaved less aggressively towards them, compared to G juveniles.
Fish were offered a choice among potential mates that differed in aggressiveness, size or breeding experience. Males did not choose on the basis of those traits. Females preferred large, aggressive, experienced males. Pairs with large aggressive males more readily kept a breeding territory, and aggressive experienced males guarded young more effectively. All females were equally aggressive in protecting their young.     

Female Choice Predicts the Best Father in a Biparental Fish,the Midas Cichlid (Cichlasoma citrinellum)

The Midas cichlid is a monogamous, biparental species. It breeds in a highly competitive system where pairs have a low probability of raising fry to independence. Both parents must cooperate to retain the territory and protect the fry from predation. Previous experiments showed that females prefer large, aggressive and sexually experienced males as mates but males do not display any consistent preferences. Here I present the results of two experiments designed to see whether qualities preferred by females correlate with increased success in retaining territories and in providing parental care. Pairs with either large or aggressive males had an advantage in appropriating and holding a breeding territory; reproductive experience conferred no advantage in usurping a territory. Aggressive and reproductively experienced males had an advantage in defending the brood from predators of fry, but size had no effect. Thus, the qualities preferred by females confer advantages both in holding territories and in protecting fry. In contrast, males need not be selective because females, once in possession of a brood, defend it equally well regardless of size, aggressiveness, or reproductive experience. The system is one of mutually enforced monogamy based on female choice; females drive the system because they provide more investment than do males (combining gametes and time) and because this investment is a reliable resource to the male.     

Factors affecting mate desertion by males in free-ranging convict cichlids (Cichlasoma nigrofasciatum)

Convict cichlid fish have biparental care for a period of about6 weeks lasting from egg laying until the young (fry) have grownto about 10 mm. However, the young can sometimes survive withcare from only one parent, and desertion of the mate and offspringby males has been observed. I tested a theoretical model modifiedfrom Lazarus (1990) which predicted that mate and offspringdesertion by male convict cichlids should be promoted by lowpredation pressure on fry, high remating opportunities for males,increasing age of fry, and decreasing number of fry. Males deserted7.8% of 334 broods studied during two breeding seasons in CostaRican streams. As predicted, males deserted their broods mostfrequently at sites with the highest brood survivorship (lowestbrood predation pressure), when fry were close to independenceand when brood size was smaller than average. Sex ratios andinterspawning intervals did not indicate any relationship betweenmate desertion and opportunities for remating for males. Thereuse of spawning caves may favor fidelity to the mate and brood,and defending the young from predators at the same time as defendingthe cave from conspecifics may favor biparental care in thisspecies.     

Behavioural responses of naive Arctic charr young to chemical cues from salmonid and non-salmonid fish

The ability to distinguish among chemical cues from multiple predators is of key adaptive value for many prey fish. We examined the attractiveness and repulsiveness of chemical stimuli from different coexisting fish species fed on different diets on the behaviour of hatchery reared Arctic charr young in a Y-maze fluviarum, where the charr could choose between two sides either with control water or stimulus water with fish odour. We used stimuli from (1) matching sized conspecifics, large (2) Arctic charr, (3) salmon, (4) brown trout and (5) brown trout fed on Arctic charr fry. Other salmonids were given pellet food. Additional fish odour treatments included piscivorous (6) pike and (7) burbot. In the control trials both sides received control water. Arctic charr young were expected to respond adaptively to the stimuli from coexisting piscivorous fish. The charr most strongly preferred water with the odour of their matching sized conspecifics, which was the only fish odour they were familiar with before the experiments. They also showed significant preference for other salmonid odours, even though these fish are potential predators on small charr. Chemical stimuli from pike and burbot, on the contrary, were strongly avoided, and burbot odour even prevented the charr to swim and enter the lateral halves of the fluviarum. Moreover, odour from brown trout fed on Arctic charr fry was avoided when compared to stimuli from trout fed on pellets. Although the Arctic charr young were completely naive regarding piscivores, the fact that they could distinguish between different predator taxa and diets on the basis of chemical cues only reflects the long coevolutionary history of these fish populations.     

Attachment of the juvenile queen scallop (Aequipecten opercularis (L.)) to maerl in mesocosm conditions; juvenile habitat selection

Substratum un-acclimated juvenile queen scallops (Aequipecten opercularis) (<18 mm and 18-30 mm shell height) were released in equal numbers onto pristine live maerl (PLM), impacted dead maerl (IDM), gravel and sand in choice chambers. Their habitat selection was monitored over a 4-day period in control and predator treatments (utilising Asterias rubens L. and Carcinus maenas (L.)). Microhabitat use of PLM by juvenile queen scallops and the presence of cues in live maerl were also investigated.In control and predator treatments juvenile queen scallops were observed to attach preferentially to PLM than IDM, gravel or sand. Juvenile queen scallops were observed to maintain a more exposed attachment site in the absence of predators but sought refuge within and between maerl nodules in the presence of both predators. Smaller queen scallops (<18 mm shell height) were more efficient at utilising maerl thalli as a refuge. Juvenile A. opercularis showed hierarchical cue responses mediated by predator presence, i.e. responding favourably to a factor associated with live maerl presence irrespective of heterogeneity in the absence of predators but favourably to higher maerl heterogeneity in their presence. If they also preferentially attach to PLM in the field, at some sites where PLM grounds cover large areas, they may thus be considered to constitute ‘nursery areas’. Habitat attachment preference appears to be predetermined and not a result of localised predator avoidance; however, habitat usage changes in the presence of predators. Maerl beds have been shown to be easily damaged by scallop dredging in Scotland and if such nursery areas are being destroyed extensively in the field, this could damage recruitment to localised adult populations.     

Selection on increased intrinsic growth rates in coho salmon, Oncorhynchus kisutch

Substantial evidence from the animal kingdom shows that there is a trade-off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast-growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild-genotype fry. In seminatural streams, fry of the fast-growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast-growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild-genotype fry were more likely to migrate than fry of the fast-growing genotypes. After being returned to the experimental streams after migration, fast-growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast-growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast-growing variants to survive under natural conditions. The large-scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.     

The Dilution Effect and Differential Predation Following Brood Adoption in Free-ranging Convict Cichlids (Cichlasoma nigrofasciatum)

Parental investment in unrelated offspring is potentially maladaptive but may be promoted by natural selection if the presence of foreign young enhances the survival of the parents' own young. We experimentally augmented broods of free-ranging convict cichlids (Cichlasoma nigrofasciatum) to test whether survival of the adopting parents' young (fry) increases, in relation to that of control broods, after the addition of smaller foreign fry, and whether such an increase can be attributed to the effect of brood dilution acting alone or to a combination of brood dilution and the effect of differential predation on adopted young. Total fry survival did not differ between experimental (E) broods and control (C) broods, but E broods had significantly more large (host) fry after 5 days and 10 days than C broods did. In E broods, small (foreign) fry suffered higher rates of predation than large fry, indicating differential predation. In E broods starting at 7.0 and 7.5 mm standard length (SL), observed fry mortalities did not differ significantly from mortalities expected from the effect of brood dilution. However, E broods starting at 8.0 mm SL had significantly lower mortalities than expected, indicating that parents that adopt smaller foreign fry can increase the survival of their own fry by the combined effects of brood dilution and differential predation. Within E broods, growth of smaller foreign fry was significantly slower than that of larger host fry, suggesting that intra-brood agonistic behaviour affects access to food for smaller fry. Therefore, increased predation and reduced growth are two negative effects that act on fry of donor parents.     

Parasitism-mediated prey selectivity in laboratory conditions and implications for biological control

In agroecosystems, parasitoids and predators may exert top-down regulation and predators for different reasons may avoid or give preference to parasitised prey, i.e., become an intraguild predator. The success of pest suppression with multiple natural enemies depends essentially on predator–prey dynamics and how this is affected by the interplay between predation and parasitism. We conducted a simple laboratory experiment to test whether predators distinguished parasitised prey from non-parasitised prey and to study how parasitism influenced predation. We used a host-parasitoid system, Spodoptera frugiperda and one of its generalist parasitoids, Campoletis flavicincta, and included two predators, the stinkbug Podisus nigrispinus and the earwig Euborellia annulipes. In the experiment, predators were offered a choice between non-parasitised and parasitised larvae. We observed how long it took for the predator to attack a larva, which prey was attacked first, and whether predators opted to consume the other prey after their initial attack. Our results suggest that, in general, female predators are less selective than males and predators are more likely to consume non-parasitised prey with this likelihood being directly proportional to the time taken until the first prey attack. We used statistical models to show that males opted to consume the other prey with a significantly higher probability if they attacked a parasitised larva first, while females did so with the same probability irrespective of which one they attacked first. These results highlight the importance of studies on predator–parasitoid interactions, as well as on coexistence mechanisms in agroecosystems. When parasitism mediates predator choice so that intraguild predation is avoided, natural enemy populations may be larger, thus increasing the probability of more successful biological control.     

Costs and benefits of biparental mucus provisioning in discus fish (<Emphasis Type="Italic">Symphysodon aequifasciatus</Emphasis>)

We tested the manner in which the number of fry influences the costs and benefits of mucus provisioning in discus fish (Symphysodon aequifasciatus) via two experiments. In experiment-1, we found that fry grew more rapidly when they were raised with parents than when raised without parents, suggesting that fry benefits from the mucus provisioning by parents. In experiment-2, a reduction in the body weight of parents correlated with brood size. The growth rate of fry did not linearly correlate with brood size. We discuss the functions, costs, and benefits of mucus provisioning in discus fish.     

Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

Identifying impacts of exotic species on native populations is central to ecology and conservation. Although the effects of exotic predators on native prey have received much attention, the role of exotic prey on native predators is poorly understood. Determining if native predators actively prefer invasive prey over native prey has implications for interpreting invasion impacts, identifying the presence of evolutionary traps, and predator persistence. One of the world’s most invasive species, Pomacea maculata, has recently established in portions of the endangered Everglade snail kite’s (Rostrhamus sociabilis plumbeus) geographic range. Although these exotic snails could provide additional prey resources, they are typically much larger than the native snail, which can lead to lower foraging success and the potential for diminished energetic benefits in comparison to native snails. Nonetheless, snail kites frequently forage on exotic snails. We used choice experiments to evaluate snail kite foraging preference in relation to exotic species and snail size. We found that snail kites do not show a preference for native or exotic snails. Rather, snail kites generally showed a preference for medium-sized snails, the sizes reflective of large native snails. These results suggest that while snail kites frequently forage on exotic snails in the wild, this behavior is likely driven simply by the abundance of exotic snails rather than snail kites preferring exotics. This lack of preference offers insights to hypotheses regarding effects of exotic species, guidance regarding habitat and invasive species management, and illustrates how native-exotic relationships can be misleading in the absence of experimental tests of such interactions.     

Use of tuber progeny tests for genetical studies as part of a potato (Solanum tuberosum subsp. tuberosum) breeding programme

A diallel set of crosses, including selfs and some reciprocal crosses, was made between 15 parents, chosen for their fertility, from those included in a tetraploid potato (Solanum tuberosum subsp. tuberosum) breeding programme at the Scottish Crop Research Institute. The progenies were grown in randomised complete block trials with two replicates at a high-grade seed site from 1994 to 1996 inclusive and at a ware site in 1995 and 1996. The parents were included in the ware trials. Tubers were assessed for visual preference in all trials and for fry colour at both sites in 1996. Emergence and maturity were recorded in the ware trials and the tubers were assessed for yield, dry matter, size, appearance (regularity of shape), scab, uniformity, sprouting in store and keeping quality. There were very few growth cracks and very few internal defects. No reciprocal differences were found. Inbreeding depression was marked for emergence, yield, tuber size and appearance, and visual preference. In contrast, the selfs had a lighter fry colour than the parents and F₁s. Combining-ability analysis (selfs omitted) identified fry colour, emergence, maturity, yield, dry matter and sprouting resistance as traits for which the GCA (general combining ability) variance and narrow-sense heritability were high enough for good progress from full-sib family selection. Correlations between GCAs for pairs of traits were examined, including those from previously published seedling progeny tests. For fry colour, an unfavourable correlation with low yield (r = 0.596) was compensated by a favourable one with high dry matter content (r = 0.652), whereas unfavourable ones between foliage and tuber blight resistance and sprouting susceptibility (r = 0.578 and 0.596) were identified for monitoring. Clones with high GCAs were identified for use as parents in future breeding and the extent to which GCAs could be predicted from the parents, and the offspring means from the midparent means, was determined by regression and correlation analysis. The offspring-midparent regression was highest for fry colour, followed by dry matter, emergence and sprouting. Values were lower for scab due to environmental variation; for maturity, yield and tuber size due to SCA (specific combining ability); and for visual preference due to both factors. The implications for a breeding strategy are discussed. Received: 28 November 1998 / Accepted: 19 December 1998