This was something of a surprise, and a very enjoyable one. The Swedish composer Ivar Hallström (1826-1901) was mostly self-taught as a composer, and the work at hand was written in 1865 to great success. Hallström and his librettist Hedberg were careful to give the work a national character, with a national romantic treatment of a semi-historical, fairy-tale subject and using folk music-like melodic themes (rather than actual folk music). The work, which achieved status as the Swedish national opera, was subtitle `a romantic operetta', but is more a singspiel than what we usually think of as an operetta (the spoken dialogue is excised from this performance, which I suppose is just as well). Every character is given a particular musical dialect so that e.g. Lisa and Peder the fisherman sing in a (virtuoso) popular song style whereas Anna - the alleged mermaid - sings in an old-fashioned ballad style (several numbers are also in the operatic style of the time - think e.g. Flotow)
The music is, in fact, catchy and immensely attractive, and there's a welter of fine, memorable tunes and atmospheric passages - and Hallström obviously knew how to put such a work together in a convincing an effective manner and sustain the listener's interest (despite his lack of formal training, his formal skills and ability to score an opera in an effective way were obviously thoroughly impressive).
The cast is young and fresh-sounding - a definite asset in a work like this - and there are many strong singers here. In particular, praise should go to the bass Staffan Alveteg as Peder the fisher; dark-toned, variegated in terms of characterization and usually genuinely beautiful. But there isn't really a particularly weak link in the cast even though one could sometimes imagine a little more vocal brilliance. The orchestral and choral performances are also very good overall, and the sound quality is excellent. Overall, this is a hugely enjoyable work with several themes that will linger in the mind for days - never a really boring moment, and even though it might not in the end be a masterpiece it certainly makes for two hours well spent. Recommended (to the extent that 4 stars might be a conservative assessment).